I was thinking earlier about, generally speaking, what makes for interesting characters. Obviously the idea of them seeming like real people is key, but to boil it down further, what makes them seem real? Is it their flaws? The great Trek characters are all pretty flawed:
Spock: Lies to himself about how logical he is, sometimes refuses to see the value in emotion
McCoy: Hot tempered
Data: Doesn't understand the emotions of others
Is being flawed/showing flaws what separates great characters like these from fun but less beloved characters like Sulu or Riker?
I don't see them as flaws. Calling them flaws implies that they would be stronger, more admirable, or generally better people without these traits, which I don't think is true. We saw this exact argument unfold in "The Enemy Within" - all these characters need their "flaws" in order to be effective at their jobs and in their relationships. A non-cocky Kirk is one without the power of command.
I think it's the very fact that so many of the characters are at peace with their traits that make them engaging and interesting. Instead of angst-ing over their flaws (Spock and Data excepted, on some level), they put them to good use, something we all can aspire to.