[W]hy is it that so many atheists are hugely impressed by the 'problem of evil'? Massimo, like so many, starts from the presupposition that the Biblical God's goodness is defined in moral terms. He predictably proceeds from this assumption to the observation that there is so much unpleasant suffering in the world that this God must be a beast or a sadist, and therefore doesn't exist.
Is it me, or is that presupposition highly questionable?
It's an interesting question why arguments from evil have become such a major family of arguments. I think it is a combination of two factors: (1) The relatively small logical space for positive arguments for the conclusion that God does not exist; and (2) the rise of the design argument.
The second of these is probably more important, but the first does contribute to an explanation of why it is always an argument from evil. On most topics, typical arguments cluster into a small number of groups, and when it comes to arguments for the conclusion that God does not exist, all Gaul more or less divides into three parts. There are positivist arguments, which argue that God does not exist in much the same way square circles don't exist; they are arguments that talk about God is inescapably incoherent. There are superfluity arguments that argue that positing the existence of God is otiose; there is no phenomenon not better explained by something other than God. And there are arguments from evil. These are families of arguments, so (for instance) not every superfluity argument will be the same as every other superfluity argument. But there are precious few other candidates for atheistic arguments, and it's easy enough to sort just about any atheistic argument available into one of these three families, depending on whether it is based on abstract possibility, principles of causal explanation, or empirically discovered defect. It's generally a good idea to remember this, by the way: not all atheists who put forward an argument from evil are putting forward the same argument from evil, or would even be convinced by the same argument from evil. An argument based on natural evil, for instance, is different from an argument based on moral evil, and both are different again from an argument based on some account of evil that accounts both, and so what refutes one argument will not always refute another, and what supports one will not always support another.
But there are things that can be said about arguments from evil, especially relative to other families of arguments. For instance, there is no secret why positivist arguments are less common than arguments from evil; logical positivism went into severe disrepute, so even positivist arguments that had nothing to do with logical positivism historically suffer under the cloud of looking like logical positivism. Moreover, positivist arguments are effectively a priori arguments, ontological arguments for God's nonexistence, and the rise of 'naturalism' has led people to be wary of a priori arguments, or, indeed, anything that looks like an a priori argument. Given the dynamics of academic life, it is highly unlikely that both stigmas will continue to stand forever; but as it is, right now, atheists who want to argue that God does not exist will tend toward superfluity- and evil-based arguments.
It's certainly possible to find people developing superfluity arguments today, and most atheists, in fact, will occasionally throw them out -- or at the very least will throw out the claim that God's existence is superfluous for explanation, even if they give no argument. But it certainly seems that arguments from evil are widespread and highly valued, and much more so than superfluity arguments. This is initially puzzling, because superfluity arguments, which are based on more fundamental causal considerations, tend to be more powerful arguments, both as to their rigor and the strength of their conclusion. And this puzzle brings us to the second point, because it is a puzzle that goes hand in hand with another puzzle: why the design argument has become so widespread and highly valued. For the two go together.
As I've mentioned before, design arguments were for the longest time almost unrepresented among arguments for the existence of God; traditionally the favored theistic arguments have been based on principles of causal explanation, not considerations about design. When you find considerations about design prior to the early modern period, it is always a remnant of Stoic philosophy. The Stoics used design arguments against the Epicureans. The existence of God or gods, as such, was not at issue in these disputes, because the Epicureans didn't usually claim that gods did not exist -- in fact, the gods were often held up as ethical exemplars. Rather, their claim was that the gods did not intervene in mortal affairs, and even more crucially, that the existence even of the gods themselves could be explained by chance motions of atoms in a void over endless periods of time. The Stoics, on the other hand, were pantheists of a sort: they held that Nature was God and God Nature and that both were in some sense Reason, and it seems to have been to argue for this view against the Epicurean view that the Stoics first began using design arguments. The appeal of this approach to someone who holds a (broadly) pantheistic view is easy to see: it's a way you can show that there is something of Reason, and thus something God-like, in the things of this world. Thus we have the design theorists against the Epicurean atomists, arguing not over God's existence but over the explanation for order in the world. You can see this sort of dispute laid out by Cicero in De Natura Deorum. And it is worth keeping in mind, not merely for the fact that a pantheist can be a proponent of the design argument, but also for the fact that, precisely because the Stoics were broadly pantheistic that their arguments for Reason as the explanation of design can be read as arguments for the existence of Reason, simply speaking, even though that does not, in fact, seem to have been the point. This ambiguity will play a small role later on.
As the old schools faded, the Christians began taking over their arguments, modifying them to suit Christian principles. One sees exactly this happening in a beautiful little philosophical work, the Octavius, by an otherwise unknown Christian philosopher from about the second or third century, Minucius Felix. The Stoic arguments are adapted to a non-pantheistic criticism of Epicureanism; whoever Minucius was, he was very familiar with Cicero and other Stoic philosophers. And from then on one finds design considerations here and there in Christian thought -- although, again, almost never, and perhaps actually never, in support of the claim that God exists but in support of the claim that the God who exists is not indifferent to His creation. That is, Stoic arguments for pantheistic providence become Christian arguments for theistic providence. But design arguments are structurally weak arguments; theists in the Middle Ages would tend to accept the structurally stronger causal arguments for the existence of God, and thus there was never any real need to put design arguments to work as arguments for the existence of God. Design considerations can be found in Thomas Aquinas and others, but why would they try put so much weight on a string when they had more robust theories of causation that could support more rigorous arguments with stronger conclusions?
The design argument as we know it seems to begin with the return of atomism, a historically Epicurean position. Most of the people attracted to atomism at the time were themselves Christian, whether Catholic or Protestant, thus they had to face immediately the fact that atomism as traditionally construed admitted of no room for providence. At the same time, the attraction to atomism went with a weakening of causal theory: if you were attracted to atomism that was a fair sign that you were losing interest in Aristotelian accounts of causation, and were, in effect, attracted to a weaker account of causation. (Otherwise you'd just be Aristotelian, accommodating atomism in the very restricted way Aristotelians have since Aristotle.) So a robust cosmological argument is no longer a strong contender, and if you were an atomist you probably wouldn't be inspired by ontological arguments, either. So the natural solution is to appeal to design. And this allows you at the same time to fill a void left by the loss of Aristotelian causation -- because the Aristotelians had developed arguments for the existence of God. And thus design can do double duty. This attitude was taken over by people we tend to think of as empiricists. Rationalists, of course, tended to accept a priori arguments for God's existence -- ontological arguments -- and thus we have a remarkable reversal. The Neo-Epicureans accept design arguments. The successors of the Stoics, however, will be the rationalists; but they will either not give design considerations a big place (continuing the medieval tradition) or, in the case of the most Stoic-like of the rationalist philosophers, Spinoza, reject them entirely.
And this is really where we still are. Thought about design arguments continued to develop. Boyle, for instance, tries to fit Harvey's work on the heart into an a mechanistic view. Harvey's science was still Aristotelian, and his research on the heart was an investigation in terms of the Aristotelian four causes, with the crowning achievement being the discovery that the heart was for the sake of circulating the blood. In a mechanistic system, without Aristotelian causal theory, this can only be fit in by treating it as functional design. Design arguments become even more widely popular with Newton, who famously gives (brief) design arguments for God's existence based on astronomy and biology. Hume has a great deal of respect for Newtonian philosophy but Hume also wants to read it skeptically, so he sets out to critique this sort of design inference, to pin down exactly how legitimate it is. He makes a special effort, however, to put it in its strongest light. To do this he draws in part on Cicero's De Natura Deorum, and in fact there are very clear, and almost certainly deliberate, analogies between the De Natura Deorum and Dialogues Concerning Natural Religion. And here the argument from evil makes a clear showing; it is the crown jewel of Hume's criticism of design-based natural religion. Most people are not particularly convinced by Hume's objections; but in trying to present the design argument in the strongest light, Hume had built a better design argument than virtually anything that had gone before, and so design arguments start looking suspiciously like slight variations of the one Hume considered. And from there we get the final succession of intriguing improvements -- Paley, Whewell, and Babbage. And after that point philosophical thought about design arguments ceases to progress and develop, and begins to alternate between deterioration and stagnation, with an occasional return to an old idea, and nothing genuinely new except the particular examples. But the design argument had by this point become 'the' theistic argument.
If, however, you are inclined to reject this argument, the easiest way to do so is simply to argue that design theorists are not consistent in some way, and that design considerations can equally show a lack of design. And you could even go back to Hume and see a sterling example of this sort of argument, so there is clear precedent. And that is the argument from evil. It is a very natural argument to use against design arguments; it requires no special theory, but simply turns the principles of the design argument, or principles that get their plausibility from the same source that those of the design argument do, against the design argument. This contrasts with superfluity arguments, which, while structurally stronger, are a lot of work and (if you are consistent in using them) tend to require much more robust views of causation which not every atheist holds. Thus the argument from evil gets its popularity from the argument from design. More than that, it is a sort of design argument. When a proponent of an argument from design clashes with a proponent of an argument from evil, what you have is someone who thinks that design can be read off objects and claims that objects exhibit design fighting someone who thinks that design can be read off objects but claims that they show no traces of having been designed. That is all.
So the simple answer as to why so many atheists are attracted to the argument from evil is that they are the sort of people who would be attracted to design arguments if they were theists. The two types of arguments make use of the same basic processes of reasoning and similar views of design as relatively obvious to spot if it's there; they just reach different conclusions because one says that this or that is obviously good enough that it has to be designed, and the other says that that or this is obviously bad enough that it couldn't possibly be designed. And both tend to be locked into the view that it is design or nothing.
Those of us, theist or atheist though we may be, who recognize that these positions are contraries rather than contradictories will tend to prefer other arguments and, as John says, regard the underlying presuppositions as dubious. I am certain, for instance, that John generally finds the argument from evil obviously dubious precisely because he would regard most design arguments to be obviously dubious; given doubts about the one, it would take an extraordinarily sophisticated version of the other to impress. But once the basic points were in place historically -- a sort of empiricism connected with the rise of science and the spread of a weak view of causation -- design arguments on the one side and arguments from evil on the other side became the easiest arguments for people to understand. So I imagine that they will be hard to put back in their place; it would require a sort of revolution in the world of thought, whereby what is now popular becomes unpopular and what is now unpopular becomes popular. We none of us have any inkling of what would accomplish that.
This is all general, of course, and there will be weird individual variations that don't fit the norm, on both sides; but it isn't difficult to find evidence that something like this account is, in fact, a correct account of the popularity of the argument from evil.