Jimmy Akin has an interesting post on one of the traditional preconditions for mortal sin, grave matter. The basic problem the post is dealing with is the distinction between venial and mortal sin; and as Thomas Aquinas says some very interesting things about the distinction, I thought I'd point them out. The locus classicus for the Thomistic view on this matter is ST 1-2.88.
The first thing to be clear about is what is intended by 'sin': Thomas follows Augustine's definition, "Sin is a word, deed, or desire contrary to eternal law," where the expression of the eternal law that most generally concerns us is natural law, i.e., the basic principles of moral reason (natural law is not, however, the only expression of eternal law; divine positive law would be another).
On this basis, Thomas considers whether it makes sense to divide 'sin' into mortal and venial. He very carefully insists that 'mortal' here is a metaphor in order to avoid arguments that rely too greatly on the ordinary meanings of the term. (He uses a great example of this. Ordinarily 'laughing' and 'dry' are not opposing terms; but if we take 'laughing' in a metaphorical sense, and use the term 'laughing' to indicate flourishing and vitality, 'laughing' and 'dry' could very well be opposites.) His suggestion is that the distinction is based on seeing sin on analogy with disease. Some diseases are mortal, fatal, in the sense that they destroy some key principle of life, or, as Thomas puts it, "in that they induce an irreparable defect through the failure (per destitutionem) of some principle." This use of the term is similar to our use of the word 'terminal', but it doesn't map exactly on that use; the idea is that some diseases are such that the body can't repair the defect, and the defect is such that if it is not repaired, the body dies. These are mortal diseases.
A mortal sin, therefore, is a sin so serious that induces an irreparable defect in the fundamental principles of our spiritual lives, where 'irreparable' here means: we have no innate means of repairing the defect. A divine intervention, of course, is still possible, but this is more a fact about divine power than it is about the sin itself. The key principle here is our ordering to our final end, namely, God as unrestricted goodness itself; a mortal sin is one that breaks our orientation toward the end ultimate goodness, whereas a venial sin is one in which this orientation is not broken but the means of expressing it are disordered, poorly suited to that end. The venial sins are called 'venial' because they allow by their very nature for venia, i.e., acquittal or pardon. This allowance may be for direct acquittal -- as when we recognize a moral failing as forgivable due to ignorance or extreme circumstances -- or for indirect acquittal -- as when we recognize that a moral failing is forgivable if a certain debt of punishment is paid. In either case, a sin is venial.
Thomas insists very strongly that mortal sins and venial sins are not sins in the same sense; the use of 'sin' here is analogous, not univocal. Venial sins are not sins in the strictest and most proper sense at all; they are condemned not because they are complete sins, but because they dispose to complete sin. Venial sins are not problematic because they are sins in a full-blown sense but because they are (so to speak) inducements to mortal sin. Venial sins dispose to mortal sins not in being sins in the same way, but in the sense that by their very nature they break down our moral immune system (so to speak), and can aggravate any mortal conditions by increasing our craving for sinning. The usual analogy Thomas makes is that venial sins are to mortal sins as accidents are to substances in Aristotelian philosophy, or in particular, as accidents that are generative dispositions. This means that, strictly speaking, there is no sliding scale between venial and moral sins; they are completely distinct, albeit intimately related, things. They are both defects of due order; venial sins tend to generate mortal sins and remove impediments to them; so in that sense we can think of a venial sin as the 'seed' of mortal sin. But this is only a loose metaphor. In reality they are related more like a bad case of flu and death. Both of these are, without any doubt, serious medical conditions, and can be lumped together as such. But the two are actually nothing alike, except in a very abstract and general sense; but they are closely related because one condition can easily lead to the other condition.
What makes a sin mortal, then? Thomas is very clear about this: any action, and I do mean any action, that in any aspect, and I do many any aspect, is inconsistent by nature with love of God or neighbor is a mortal sin. Venial sins are actions that are not inconsistent with love of God or love of neighbor as such even though they are poorly proportionated to it. Malicious hate speech, for instance, is inconsistent by nature with love of neighbor (obviously) and love of God (insofar as human beings are in the image of God). It is a mortal sin. Going too far in making jokes is not in its nature consistent with love of neighbor or love of God, even though it is poorly suited to both. If there is no malice in it, it is a venial sin. Venial sins tend to be things OK in themselves that are either not taken far enough or taken too far; the additional condition they have to meet is that the person committing them must not be treating them as means to a mortal sin or as ends in themselves. Thus there is nothing wrong with sexual talk under proper conditions (e.g., marriage) when it is done moderately and rationally; therefore sexual talk is only a venial sin outside those conditions or when done immoderately or irrationally; unless it is done for the purpose of some other grave sin (like adultery) or simply because it is a sin.
Augustine has a scenario in his Confessions that conveys the idea quite well. When he was a boy, he used to steal pears from someone's trees. He goes on at great length about those pears, which has led some people, like Bertrand Russell, to mock him for it. But he's making a serious point. There's nothing wrong with taking pears from someone else's trees without their permission under certain conditions when it's done in a certain way, e.g., when you need the pears to feed your starving family and have tried to get permission; there is something wrong with stealing the pears to give them to someone who you know doesn't need them, or even because you really like pears; there is something very wrong with stealing the pears precisely because it is stealing. And Augustine's point was that, in fact, he stole because it was fun and exciting to steal. When he stole the pears, he threw them away, because he didn't care about the pears at all. Augustine doesn't frame it in terms of mortal and venial sin, but that's the distinction exactly.
This account, I think, deals very easily with the sort of problem raised by Akin, namely, that there are not sharp lines in identifying the gravity of the matter. In fact, if Thomas is right, and there's good reason to think he is, there are only two things that count as grave matter at all: strict inconsistency with loving God and strict inconsistency with loving neighbor. Everything else is mild. Although, of course, to say 'everything else' makes it sound as if mortal sin were rare, when it clearly is not. And note that it doesn't matter what the external behavior in question is. Giving alms to the poor can be a mortal sin if it is done with an intention irreconcilable with love of the poor. Drunkenness is a mortal sin if someone drinks in order to disorder his reason; it is a venial sin if it someone gets knock-down drunk merely because they misjudged how much they were drinking. In the first type of drunkenness you are to blame for the drunkenness itself; in the second type you can only be blamed for not being careful enough. In unusual circumstances, murdering someone may well be merely a venial sin. This doesn't mean that it is any less serious; it doesn't mean you can't be blamed for it. In English we have a useful expression for the difference; we can say that such-and-such is wrong but 'forgivable', i.e., the circumstances and nature of the act are such that there's nothing in it that prevents us from going easy on the person who did it (either straightway or whenever they meet some condition). That's more or less what differentiates the mortal from the venial: venial sins are wrong but 'forgivable'. Mortal sins, however, are not 'forgivable' in this general sense; they can strictly speaking be forgiven, but there is nothing in the act itself that is open to it. Forgiveness has to come in as an extraordinary intervention that introduces a radical transformation. Venial sins are illness, sometimes very serious illness; they require recuperation, sometimes medicine, sometimes surgery. Mortal sins are death; they require resurrection.