All of these things irritate me.
1) Flaws that aren't flaws. Either these are the kinds of things that you know and the readers know aren't really flaws- "She's too nice!" or "He's too modest!"- or they're things like, "Well, she bites her nails!" Those first are the kinds of things you tell people who want to know what your greatest flaw is. The second are bad habits that can be overcome- which leads me to
2) Flaws that can be airbrushed away. I've lost count of the number of times I've read a character profile like, "Timid, but learns to be courageous by the end of the story." And if her flaw vanishes, what happens to her humanity? Some published novels come close to this by having their characters apparently earn things through their suffering, but the author is wise enough to end the book after that, since it would mean having to write about a character who was nearly perfect. Perfect people, or those who become so, are boring.
3) Flaws that don't inconvenience the character. If she isn't good at dancing but never has to dance... yeah, what was that about it being a weakness again?
4) Flaws that are always played as strengths. There's a fascinating commentary to be done on things like having a quick mouth, a quick temper, or quick wits as both flaws and strengths, but too often the character has them only as strengths. I've read smart-mouthed characters I wanted to smack, but everyone else stood around them gaping in awe. Similarly, the characters with quick tempers often don't get in trouble for losing it, and the characters with quick wits never jump to the wrong conclusions.
What I recommend
1) Let your characters make mistakes. This is something that most amateur authors, and even some published ones, seem really afraid of. Everything that happens to their character is the parents' fault, or the siblings' fault, or an outside circumstance's fault. People do make mistakes on their own, you know. Let your character change as the result of what he or she does as well as what happens on the outside.
2) Give the character some real flaws. I've heard people worry about making the character unsympathetic, but audiences have varying tastes. Some people really sympathize with people who whine about the ordinary problems of life. I don't, but that's me. And if you write well enough, people will forgive you just about anything.
Some flaws that are not so easy to turn into showings-off of virtue include:
hypocrisy
being careless with money
forgetting important things (dates of birthdays, the locations of important objects)
being aggressive and overprotective
condescending to others
constantly turning the conversation into one about themselves
complaining about people behind their backs but being nice and sweet to their faces
insisting that others share their obsessions
laziness
assuming the rules don't apply to them and screaming when they do
self-righteousness
fanaticism on a particular topic
All of these are harder to turn to advantage, and, of course, most people are somehow afraid that their characters will magically become unsympathetic if they use them. No, they'll sound like real human beings.
