Commenter David mentioned the following in a recent post, in response to which I promised a later follow-up:
Left-wing political correctness embraces very simple values that were once conservative values as much as liberal ones: talk to people using respectful language whether you agree with their actions and attitudes or not. Do not stereotype the actions of a whole group by those of an individual.
I've actually heard this argument before. Suffice to say, I'm not convinced that political correctness is simply niceness. The best evidence for this is that we use two different words for them; if PC were merely niceness and nothing else, we'd have no reason to have invented a new phrase.
The context in which I referred to PC in the earlier post was the sense used by Bruce Charlton in his new book, Thought Prison: The Fundamental Nature of Political Correctness, by which he simply means leftism in general (or, perhaps more specifically, the leftist/modern philosophical project). David clearly means it in the sense in which it is more commonly understood today, to refer to a program of speech controls enforced by social sanction.
So what is PC if not identical to niceness? Is it deficient in some respect, or does it add something to niceness that niceness itself does not require? PC is similar to niceness in that it exists to limit respectable discourse and thus to protect the existing social order from excessively severe attacks. That is where the resemblance, I'm afraid, really ends.
For where niceness is concerned with protecting a social order concerned with community, PC is concerned with protecting a social order that is explicitly anti-community (indeed, one that parcels up community into competing and hostile groups, some of which are entitled to PC protection and others subject to explicitly PC nastiness). Both require conformity to socially-established norms but order these norms toward different ends. The order which niceness seeks is fundamentally cooperative, communitarian, and traditional; it is pious and humble. Political correctness seeks an order that is noncooperative, individualistic, and revolutionarily novel as a matter of principle. It regards desecration and shock as a means to that end.
PC is therefore a direct competitor to mere niceness; both seek the protection of a social order, but the social orders they envision are irreconcilable. Niceness has no interest in protecting the manifold absurdities of modern liberal society. Political correctness has no interest in what it sees as the stultifying, arbitrary, and suffocating rules of traditional society.
(In fact, in PC there are not even really "norms" in the strictest sense of the word. PC nominally proscribes racist speech or actions, yet these are officially PC-approved when the offender is a higher-status victim group than the race being slandered, as when California homosexuals exploded with savage racist rage following the passage of Proposition 8, banning homosexual marriage, with strong black support. The politically correct, being utilitarians and consequentialists all, see the end as universally superordinate to the means, so that grossly un-PC behavior is encouraged provided it culminates in a more broadly PC gestalt). There are plenty more examples of PC being not kind but still definitively PC in the strictest sense of the word; a casual Googling reveals many.
PC also reacts in comparatively more severe ways to violations of its sanctions than do the merely kind. Because kindness is simply a disposition whereas PC is an institutionalized ideology, violations of the former are treated with, at worst, coldness and avoidance where violations of the latter are subject to often quite devastating and disproportionate retaliation. PC is therefore far more overtly coercive than mere niceness.
In their respective extremes, PC and niceness differ again. Niceness taken too far is deficient: either in justice (e.g., allowing injustice to occur because one is too nice to intervene) or in charity (e.g., allowing a person to persist in bad habits because one is just too darn nice to correct them) or in courage (e.g., allowing oneself to be bullied by others because one is too nice to stand up for oneself). PC, taken too far, is excessive, ruining people's lives and pitting entire cultures against one another. It is the injustice, the uncharity, and the cowardice against which excessive niceness is too feckless to do anything.
It goes without saying that, while both niceness and PC proscribe certain behaviors and manners of speech, PC's scope is comparatively limited; it protects with greater intensity many fewer people (and does not because they are people but because they belong to the groups they do), where niceness protects everyone. Niceness prohibits meanness; PC prohibits insensitivity.