Greenout - she's too nice in the sense that she won't do anything it takes to win. Serena and, moreso, Justine will do anything to win - stare down their opponent, cheat, take conventient breaks, yell at an umpire, etc. It's representative of how those two will dig deep to will a win. Kim just stays pleasant and sort of does her thing - if it works, good, if it doesn't, life will go on. She doesn't need to win, and that is why she can't dig inside herself in these big occasions. It's in that sense that she's too "nice" to win.
bandabou - there were a number of matches from 2000-2001 that Serena choked/got tight in (am thinking in particular of her matches against Jennifer and against her sister).