The trick, much like with character backgrounds, is to start sparse. A good character should be easy to fit inside a three-beat structure -- very broad background, core competence, and core flaw. It has a double benefit in making your character easy to summarize to new people while forcing you to focus on what the character really can't do well. When you have only one real flaw to list, it tends to be a big one, and that means stretching your creative muscles to make said flaw as interesting as possible.
But what kind of flaw should you include? The simple answer is the one that makes for the most interesting roleplaying, but that's a bit like saying that you should win a race by being a better driver. To my mind, there are five broad sorts of flaws you can add to a character that really add some spice -- ironic, tragic, ignorant, redemptive, and functional.
Ironic flaws are exactly what the name suggests, and they're the type I come to most frequently. They're also easy to identify, because they just require you to take some core assumption built into the character concept and reverse it. So you wind up with an expert thief who can't shut his mouth after a successful heist (thereby alerting anyone and everyone to what he's just done), or an aggressive and hotheaded superhero whose powers are purely defensive. Creating an obvious dichotomy between the character and his biggest flaw makes the character easy to get a handle on, and it sets up a simple internal conflict that doesn't require a whole lot of additional work.
Of course, if your characters all have ironic flaws, it starts to feel like you just like seeing people who are bad at their jobs. So you'll want to mix things up a bit.
Fortunately, tragic flaws are both the opposite of ironic flaws and equally simple -- they're a character's virtues writ large. Everyone points to Hamlet when discussing tragic flaws, at least if you've dabbled in any literature coursework, but there are other examples, and it's a simple concept. You might be playing a Science officer in Star Trek Online with a penchant for xenobiology -- but she's so obsessed with the science that she doesn't regard other sentient creatures as people so much as bundles of nerve clusters. Or perhaps you've got an aging and dedicated warrior who really only has the one skillset after years of practice, meaning that he defaults to solving all of his problems with violence.
Ignorant flaws are another turn away altogether. The easiest way to understand them is to watch a few zombie movies and notice how in every single film, none of the cast members has ever seen a zombie movie, thereby losing out on any sort of helpful cultural osmosis or even just giving characters a handy name to call the walking dead. It's not that the character isn't smart, necessarily, just that she subscribes to a model of the world that isn't entirely at ease with the way the world actually works. Done right, you wind up with a character who, say, honestly believes that heroic fantasies are the rule of the day and has no coping mechanisms when that fantasy breaks down (Ironfist in Last Stand of the Wreckers, for instance). Done wrong, you wind up with Dana Scully, a character made stupid by the necessity of maintaining a worldview without any logical basis.
Redemptive flaws also take a different tack, and they're also ripe for abuse. The short version is that the character has done something very, very bad, and as a result he's trying to compensate for past mistakes, either by hiding his actions or working overtime to make up for them. Of course, this is also a road fraught with characters whose Dark Past is not actually all that dark, and it's all too easy to make a character who angsts and whines about a mild transgression.
The key to making this sort of flaw is to up the ante and make it a conscious decision. Your character didn't, say, fail to protect civilians in a war -- he was right in there killing and didn't even question his choices until later. Don't make the horrible past event something over the top; make it real and palpable and something worthy of feeling awful about. Look at it from another angle: If your character's main struggle is to realize that he shouldn't feel so bad about what he did, you're doing it wrong.
Last but not least, functional flaws are simply frailties that have nothing to do with personality, occupation, etc. You can be struck blind whether or not you're a powerful spellcaster, for instance. Again, much like redemptive flaws, functional flaws can too easily fail to impact a character. Physical inability is a functional flaw (blindness, deafness, being stuck moving at a pace no faster than a slow shuffle). Serious mental disorders are functional flaws (such as being unable to focus on one topic for more than hey look a bird). Not knowing how to program your VCR is not a functional flaw, unless you're playing EVE Online and your VCR is somehow hooked up to your ship's weapon systems.
Functional flaws are attractive to younger players or people new to roleplaying, afraid of what might happen if a given character somehow falls short in a crucial area. That doesn't meant they can't be useful; it just means that first you have to realize that the purpose of any and all flaws is ultimately to strengthen the character. The bigger a handicap, the more likely a character is to stick in the minds of other players, and the easier it becomes to flesh a character out from the broadest strokes into a more dynamic personality.
I've got a lot more to say on this particular topic, but I'm going to be responsible and just leave this as an area we'll be returning to in the not-so-distant future. Comments are welcome as always, whether you prefer the comments below or just shooting a message along to firstname.lastname@example.org. Next week? I think we're due to take a look at an archetype that knows what he wants and just how to get it.
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.