I spend a significant amount of time in my Creative Writing class each semester, talking about characterization. Primarily, about giving characters flaws and/or quirks. Doing so makes them multi-dimensional and, more importantly, identifiable to the reader. Nobody likes a "perfect" character. How boring!
Here's a wonderful blog entry I found today, about this very thing. I like how the author draws from classic literary characters as examples. Enjoy! The Unique Appeal of Flawed Characters