Besides the already mentioned factors I also think that envy is causing a lot of the hate. Caro's rise to staredom has been incredibly quick: At the beginning of 2008 she was still only ranked like #60 or so! And her rise from # 10 in the world to # 2 took only 10 months, which is quite rare (although done slightly quicker by Sharapova and Ivanovic, who were 28 and 14 days quicker respectively).
Usually the WTA players progress much more slowly as the come nearer to the top 10. Even players like Venus, Serena, and Kuznetsova needed more than three years to reach the #2 spot in the rankings from the time when they cracked the top 10! And Dementieva only reached her career high ranking (#3) in 2009 - 8 years after she cracked the top 10 in 2001. Mauresmo needed 5 years to progress from #10 to #2, while Safina and Henin needed just over 2 years.
The usual 'slow' progression means that the players have a lot of time to establish themselves in the top of tennis. They gradually work their way to the top by winning bigger and bigger tournaments and producing consistently good results (and thus 'deserving' their place in the top).
Caro, on the other hand, came like out of nowhere! "where did she come from", is what many tennis fans (who have not followed her career closely) seem to think. "What did she do to get there and does she even deserve it?"
I guess the 'Caro haters' only wish, that their faves would have the same success as Caro! Furthermore: Caro is blonde, young, good looking, sweet, friendly, well liked, rich, succesful, gets a lot of media exposure and fine sponsor deals, which are all perfectly legitimate reasons to cause envy and hate