What Data didn't realize was that he was perfect the way he was.
Well, 'perfect' may be over-stating it, but it would've been awesome to see him realize he was equal--that being different did not in any way make him less. That would've been a powerful statement for all of us in the real world who are made to feel less for thinking and feeling differently too.
*sigh* Well, that's what's fanfiction is for, I suppose.
ETA: I agree Data as a real human would've been terrible. His differences were what made him so special and likable. Just too bad he never seemed to fully realize that.