I think many of the concepts tossed around here could work as a premise for a character. The real question is whether or not the player ever gets beyond the irony of the unusual design. It's one thing, for example, to design a half orc bard. But beyond the shock of the incomensuables, comes the challenge of making the concept work. The player has to understand that for the character that combination must have somehow worked in his life. He must have chosen his career and the reason for that choice must have been more substantial than the generation of a single rimshot. There has to be a plausible rationale for it. If he is continually playing up the irony and never gets past the this-shouldn't-have-happened mode, then it's lousy role-playing after all. Likewise it's bad role playing if the irony of the concept is continually the main point of the play. Somewhere along the line, the character will need to express an interest in other things and develop a personality with a litte subtlety to it. He will have to express a taste in clothes, liquor, choose a style of music or poetry, develop a relationship with each of the other characters in the party, etc. And all of this will need to draw from a rich personality design, not simply the gag value of the initial concept. The question is simply whether or not the player has the maturity to make it work.
My own pet pieve is players who role play their characters doing really stupid things on the bases of some personality premise. I'm talking about severely decreasing survivability on the basis of a premise about the character's personality. This is often played up as role playing, and I've met some players who seem to pride themselves on their willingness to do things contrary to the interest of the character because it fits with their character design. This works to a point, but often I think players forget one thing, if it's amusement to them it's often life and death for their character. And life and death situations have a way of paring down personality to suvival instinct. So, no, I don't think the Rogue will be spending several rounds stealing stuff while the party is on the verge of losing a battle (unless he could run perhaps), and the wizard with the fetish for colour spray isn't really going to be casting that when he should be letting lose with a fireball, and the fighter isn't really going to be doing a victory dance ove rthe first giant while the second smashes the cleric he'll be needing after tha battle is over. To me good role playing would be imagining what these characters would do to survive and accomplish their goals given their actual personality, not contriving actions which virtually ensure death and pretending that is the test of good role-playing.
I do think that sometimes players expect mercy. It's as though a winning personality means that I as the DM am supposed to hold up the laws of (meta-)physics for the amusing character.
Edit: Oh geez! Sorry about the Thread Necromancy. Forgot how I found this one and how far back the discussion seems to have ended.