It's useful to understand that you could make your character profoundly incapable of understanding selfish or deceptive motives (ignorant flaw), or make him a former murderer who's adventuring as a sort of work-release program (redemptive flaw), or even just a deaf mute (functional flaw). But it's important to understand why these things matter -- in a world where everyone is telepathic, a deaf mute is only at a slight disadvantage. You need to pick out marquee weaknesses that are relevant, and you need to know when a smaller flaw is actually useful to roleplaying.
For the purposes of this discussion, I'm referring back to the first column advocating that a character should be summarized in a quick three-beat that includes a flaw. Whatever flaw fits into that structure is the character's marquee flaw. (To use an official example -- "Koss is a Kournan member of the Sunspears. He's a valiant warrior with a knowledge of battle and an easygoing personality. However, he doesn't like to work on the right side of authority and winds up rubbing many of his superiors the wrong way.") Minor flaws, meanwhile, are the sort of thing that gets woven in over the course of gameplay.
Once you have an idea of the various flaws that a character could have, your big choice is to figure out which ones you actually want to play with. And that's a long-term decision, since it's the sort of thing that shouldn't actually be corrected. Barring a complete character rework, your character's biggest flaw should be in place throughout play.
This is the other area that frequently trips up new roleplayers. When you're first getting into the role, you find yourself thinking of flaws as something you'll eventually manage to overcome, especially if you're younger and/or coming to the tradition from tabletop play. (For those of you who have never taken part: Many pen-and-paper games allow you to make your character flawed in some fashion in exchange for a more powerful character. So you might be a midget with explosive flatulence and a third foot growing from your forehead, but you have great attack rolls.)
However, a flaw is not actually an impediment to the player, just to the character. And the goal when you pick a flaw isn't to minimize obstacles, it's to maximize them. You want to throw just the right sort of obstacle in your path so that when your character achieves a victory it's more significant.
So the best flaw for each character comes down to two aspects -- what will make the most interesting stories, and what best suits your playstyle. The latter is something that no one else can really tell you about, since no one knows better than you what you like to do. But each type of flaw works in different sorts of stories and creates a different impression. Ironic and ignorant flaws, for instance, are pretty much de rigeur for comedic roleplay, since incompetence and blithering stupidity are the easiest way to play anyone for laughs.
In brief: A character with an ironic flaw is going to have his greatest triumphs when he succeeds at some core aspect of his field despite his inability. One with a tragic flaw succeeds when he manages to rein himself in just enough, most often when he doesn't do something that would lead to problems down the road. Ignorance-flawed individuals find their greatest victories in seeing through their own deceptions and treating the world for a moment as it is rather than how they want it to be. If your character has a dark past and is trying to redeem himself, he'll just be happy when he does something that is unambiguously not what he would have done before. And the functionally flawed are triumphant when they score a victory, period -- they have enough stacked on their plates that the little things merit a pat on the back.
If you know the group of people you'll be roleplaying with, you have a good idea of what sort of stories they like to tell. That's certainly easier to tailor your character's flaws around... in a sense. It could also mean that you know certain people will always pick certain flaws, so you have to avoid that territory to avoid running with the Dark History Brigade. If you're not sure whom you're going to be playing off against, however, the best you can manage is to try to create a character that can work in a number of different groups, with a flaw that supports the type of adversity you want to face.
The game in question also plays an important role. Certain flaws are more or less significant in roleplaying -- the aforementioned deaf-mute, for instance, would be hard to play simply because it would be difficult to make it clear to others what sort of character you were going for. I've seen it done, and it is possible, but it requires a very gentle hand and a lot of patience. Having a bad leg, on the other hand, would be pretty trivial because of the way most game engines are designed.
Last but certainly not least, all of this is true of marquee flaws, but you can have issues without their defining a character. Sure, your warrior may have killed people for money in the past, but she is also afraid of drowning, hates the taste of potions, and is aggressive in the extreme. Minor flaws are best woven in piecemeal, here and there as you play, just like backstory. These are the sorts of issues that a character might well wind up overcoming over time without cheapening character identity -- or the originally minor flaw can wind up becoming more defining than the marquee flaw, which slowly fades to the background and gets resolved.
Ultimately, flaws are the part of characters that makes roleplaying awesome, that makes characters memorable in so many ways. The best way to get a grip on them is to read through your favorite book or watch your favorite film again, taking pains to notice each time the main character screws up. It's not that he or she is incapable of failure that makes the story great -- it's the fact that things are accomplished despite those failings.
Feedback on the column can go in the comment field or off to the magical land of email, as long as the latter is addressed to firstname.lastname@example.org. Next week, patterns would suggest that we're doing another archetype, but I'm going to take a break from format and undermine everything I have said here up until this point.
Every Friday, Eliot Lefebvre fills a column up with excellent advice on investing money, writing award-winning novels, and being elected to public office. Then he removes all of that, and you're left with Storyboard, which focuses on roleplaying in MMOs. It won't help you get elected, but it will help you pretend you did.