Serena will hit some errors but she hits winners too so this is not too big of a weakness. It normally balances out. So looking at the matches she lost, why did she lose them?
2002 Kim in LA
2002 Justine on clay
2002 Patty Schnyder on clay
2002 Chanda rubin on hard courts
In the Kim match Serena said she was tired from playing Jennifer the pervious day indirectly. I think Chanda caught a rusty Serena and mixed in net play. I did not see the Patty and Justine mathces but what is amazing is other than the match where she was "tired," all of these ended 7-5 or 7-6 in the third set which means that Serena is not normally going down without a tough fight.
so here is my short list:
Serena's Weaknesses (WTA PLAYERS PLEASE READ THIS)
1 might be rusty and/or tired
2 can be rushed from the net
3 on clay maybe she loses some power
4 hitting down the middle hard to her feet can get her off balance
5 can make too many errors
not too much to work with...