The object of the Magisterium, exercised by the Pope and his bishops (with guidance from Vatican dicasteries such as the CDF), is to settle arguments over doctrine that might otherwise plunge a billion Catholics into endless energy-sapping disputes and parties. It's the task of the teaching authorities, and theirs alone, to determine who is in and who is out. Once they have declared someone to be dissenting from church teaching, then to call that person is a dissenter is a statement of fact; but if they haven't, no Catholic can assume the right to do so.
While I sympathize with Austen Ivereigh's problem with people being eager to label people who diagree with them as heretics, dissenters, or worse, I don't think this is the right response, simply because this is not a plausible account of the Magisterium. Despite the continual use of the Latin word in capitalized form, the Magisterium is just the Church itself insofar as it teaches authoritatively: and teaching, not settling disputes, is its object. Indeed, many of the more important exercises of teaching authority have been cases where the Magisterium explicitly refused to settle the debate, and simply told the parties in question that neither had the right to pretend that it was settled: the Molinists and Thomists (or Banezians, as we often call them today) being one obvious case. Moreover, the primary responsibility for resolving disputes -- nipping them in the bud before they "plunge a billion Catholics into endless energy-sapping disputes and parties" -- really belongs to the people disputing. The Magisterium only gets involved with disputes when the people involved have fallen down on their intellectual and moral responsibilities. Waiting around for the Magisterium to step in and resolve things, besides being extraordinarily passive, is itself more likely to guarantee the proliferation of energy-sapping disputes than anything, because it guarantees that nothing will be resolved until the bishops actually do step in authoritatively: it is a recipe for cold war, not for peace, a policy that will lead to people constantly sniping at each other when they think they can get away with it, and each side taking the fact that the Magisterium hasn't resolved the matter definitely as evidence that it need not worry about whether the other side's accusations have a grain of truth. In short, while I can see why someone would think that this would further the peace, what it would actually do in practice -- what it actually does when put into practice -- is merely change the tone of the dispute from aggressive to passive aggressive.
The real problems arise from two factors: (1) people with an agenda latch on to any rhetorical advantage they can get, regardless of how loosely it fits; and (2) some people, once they have labeled something, simply refuse to let it go, and nothing will convince them. With regard to (1), we get the people who are always on a heresy hunt. After the Syllabus of Errors there was an increasing trend of people trying to pin their opponents with the label 'Modernist' on even the slightest pretext; a trend that grew worse and worse until Benedict XV started putting on the brakes. Such people we always have with us, unfortunately. On (2), the Jansenists, mentioned in Ivereigh's post, are an interesting case in point, given how much criticism bishops have taken for criticizing theologians recently. Jansenism, despite the trouble it got into, was not a particularly odious set of errors, contrary to modern-day popular belief; the distance between them and their Molinist opponents were not originally so great as it was when the dispute came to an end, and the more serious periods of dispute were often due more to politics than anything else. But Jansenism did get a very bad reputation in part because Jansenists refused to be corrected, so that the crackdown on Jansenism eventually became very severe. Why? Because, while they insisted that they were completely faithful to the Magisterium, every time it issued anything correcting them, they simply reinterpreted the correction (sometimes quite ingeniously) so that it just didn't apply to them. The first time or two one could treat charitably; after all, nothing in Catholic doctrine implies that the Church is infallibly clear, and nothing implies that bishops always have their facts right. But it became very clear that this was the result of just a refusal to consider the possibility that they might be wrong; and it was arguably this, more than the particulars of their theology (which, after all, primarily only erred by taking some things said by Augustine a little too strictly), that lead to the harsh rebukes Jansenists later began to receive. The worst cases, of course, come from those who fit into camps (1) and (2) both.
Magisterium is not a complicated concept; it really just is a matter of learning and teaching, and only if this is understood can Catholics really have a reasonable approach to the Magisterium. The proper student is not the one who keeps insisting that no one knows the answer until the teacher has given it (those are the students teachers usually want to shake); the proper student is the one who uses her God-given brain to figure out the answer, always being open to the fact that the teacher might point out her mistakes at some point. The correlative to teaching is not passive obedience but study. If people really think something is heretical, dissenting, or just plain wrong, they should say so -- they should just usually say it as fellow students. It's no different in this respect from the natural magisterium of reason, to borrow a phrase from Rosmini. Saying that something is heretical, for instance, is just saying that it is irrational once one assumes the principles of faith; to that extent it's not fundamentally different. If you think something is irrational or wrong, as a student of reason you should feel perfectly free to say so, and defend your reasoning to this conclusion; you don't have to wait for the absolutely conclusive demonstration. But when you lack the demonstration, you should also not pretend to have it, and should be willing to discover that you simply missed something important; and you should never, ever insist unequivocally that something is irrational or wrong without proper study. In other words, you should distinguish between times when there's a very good case to be made that the position really is wrong or unreasonable, and times when you just think the questions need to be raised.
In connected news, Cardinal Wuerl and the Committee on Doctrine has issued a statement (PDF) on the Committee on Doctrine's recent criticism of Elizabeth Johnson's The Quest for the Living God, which discusses the concept of Magisterium. It makes an interesting juxtaposition to Ivereigh's post, because while Ivereigh's post essentially puts all the responsibility on the bishops, many of the criticisms the Committee on Doctrine received imply that the responsibility should be taken away or sharply restricted. Wuerl deals with some of the least reasonable arguments against the bishops' actions (the ones along the lines of 'How dare they do this at all'), although it doesn't defend the content of the report on Johnson in any way. Of course, the people who won't be convinced still won't be convinced that the bishops should have anything to do with the matter; it's interesting to consider many of the comments in this thread and think about what they would actually mean in terms of the teaching of the Church. In my own opinion most of the comments are proof that Glubglubglub is not merely the language of theologians; they are essentially proposals either for incoherence or for episcopal abdication of responsibility. The fact of the matter is that it is simply Catholic doctrine that bishops have the right to criticize theologians, and that everyone else has the responsibility to take such criticisms seriously by looking into ways in which the bishops might have a point. If you think the criticism imprudent or incorrect in some way (both of which do happen), you are perfectly free to say so, and to say why, and to try to convince the bishops of this point. But it just makes a hash of Catholic thought to suggest that they don't have the right in the first place to make any criticisms they honestly see fit to make, or that theologians have some special privileges in the Church that don't derive entirely from their actual value in elucidating Catholic thought. The bishops should certainly try to get everything right; but there's nothing demanding that they have to cross every t and dot every i: if they come across something they think could confuse the faithful, they have a responsibility to say so. They say that people once made fun of Nestorius by chanting "Strictly speaking, strictly speaking," whenever he went by, because that's how he'd answer any criticism of his views. Even if they don't get everything right "strictly speaking," even if they misread, the fact that they can misread in the first place, the fact that their interpretation is off, should give theologians genuinely interested in teaching Catholic thought at least enough pause for them to ask what they need to clarify so that they won't be misunderstood. The fundamental characteristic of the Magisterium is that it teaches, and the fundamental characteristic of individual bishops insofar as they participate in the Magisterium is the same: so a person's fundamental response to those participating in it should be to learn, even if learning leads in a different direction than the bishops themselves expect. Again, the correlative for teaching is study; study is the way one learns when taught. Even when the particular teacher in question is wrong in some way! Especially if you think the teacher is wrong in some way!
But it really does seem like Catholics fall into two groups; those who want the teacher to do everything and thus will not study; and those who want the teacher only to say things they already know and agree with, and thus will not study. Neither group is being a good student; indeed, both groups are simply refusing to be students at all. This is not healthy.