Oh, not COMPLETELY bad. Just bad-ish. Have some bad-esque qualities. Otherwise, they're completely perfect. And we don't like completely perfect people. Know that girl who is/was acing your history class, answered every question, and knew just how to wear a canvas sack so she could jump on the runway at any moment? Yeah, you hate(d) her. Don't try to deny it.
So, then, if readers are supposed to sympathize with your characters, one surefire way to make that not happen is to make your main character perfect. Because, be warned, if your main character (or the love interest) comes off as perfect, she/he may and probably will be subject to criticism and parodies like this. Also, the plot will probably drag, due to the lack of character arc. (Unless, for some reason, the character starts out perfect and turns into a really horrible person. Now that would actually be interesting...)
Also, it usually helps if the flaw is related to the plot in some manner. And while "poor eyesight" is interesting, it and those like it do not really count as "flaws." They're just...things.
An example of a plot-related flaw:
Character has a really poor sense of direction. Then she discovers a labyrinth (like from Greek Myths, yeah?). Not only does this flaw allow for possible humor, it also adds to the conflict of the story.
Flaws aren't exclusive, either. You can have more than one. In fact, do! More flaws = more interestingness. Just be careful on overloading--we don't want readers wondering how your character has managed to even stay alive for x years of his/her life.
Characters making mistakes is worthy of its own post--watch for that one later on.