Zvonareva was in slam finals, a YEC final, won IW and was a YE #2.
She's not awful. Its just that now it looks like Caro had a small window, so the Zvonareva SF and Li SF now look like big losses.
But in reality, Kim played very well in that USO final. Maybe the Li loss with matchpoint was closest to a slam win, because Kim never played the AO as well as the USO.
But every good player has some slam losses at least as bad as losing to Zvonareva.
The problem is that getting to a slam semi to face a nonslam winner seems far away now
Of course Zvonareva is not a gimme, neither is Na Li, but these are the types of matches you face at latter stages of grand slams that are very winnable (for both players this is true), and are disappointing if you don't win them. The Zvonareva one specifically, this was Caro at her very best, she came in having won 3 titles in the last 4 weeks, including beating Zvonareva 6-3, 6-2. Her performance vs Zvonareva in the USO though was abject, and far from her best. Did it have something to do with having played 4 weeks in a row on 2 different continents coming in? Who knows, but Caro was very flat this match, something you can't afford in grand slam semi-finals.
You highlight the problem anyway. Everyone has these losses, it's just for most players they aren't career defining moments, because they also won plenty of these matches and probably won grand slams. If Caro ever wins a grand slam, these losses will be forgotten, because eventually she got over the hill.
Anyway here are some comparisons for their best year results at grand slams from recent times:
Caro QF, R4, SF, SF (17)
Safina SF, F, F, SF (22)
Jankovic SF, SF, R4, F (19)
Zvonareva F, F, SF, R4 (20)
Dementieva QF, SF, SF, SF (19) [I know she made 2 finals in 2004, but it was in non-comparable times]
These are the most famous players who haven't won grand slams I can think of in recent times. This highlights in general that Caro has under-performed at grand slams.