You probably already know that a character with too many flaws can be an unlikable loser, whereas a character with too few can be an annoying Mary Sue. I talked earlier about how, like the number of flaws, the number of times your character is right about their guesses can also make them a Mary Sue. There is a third criterion that, like the second, is often ignored.
If your character lets the plot take them along, relies solely on others, and never solves their own problems, they can be boring and annoying. If they never get themselves out of their own situations, they range anywhere from completely unnecessary to the plot to a burden on both the readers and the other characters.
On the other hand, if they get themselves and possibly their group out of every situation single-handedly, their Sue-dom starts to show. With all of the troubles they will face in an average story, having them at the center of every solution is unrealistic. This counts even if other characters execute the solutions, but one character is always the mastermind. Other characters and external forces should make major contributions to how your character gets out of many situations.
Balance is the key. Too self-sufficient in every way and you have some form of a Mary Sue. Too much on the other side and you have a needless burden on the plot.
The follower of the day is namedfortheflight.