A lot has been said about the 3rd and 4th seasons, saying that they pale in comparison to the original two - that they're terrible, even.
Personally, I think the hate is overblown. As with many things, fate just decides to make perfectly good/decent/average works into whipping boys for no reason. While the 3rd and 4th seasons weren't as good as the original, they were still decent enough in their own right.
I don't really care much about the art style change; this may be because it's what is used in EVERY piece of Dexter-related merchandise since then. The character designs didn't really look out-of-shape to me either. Stylized, yes, but it didn't matter to me.
And as for one of the biggest complaints about these seasons - Mandark's origin apparently being "retconned"... well, I don't see it like that.
First off, Astronomenoff is his last
name. We also never saw his parents in the first two seasons, at all
. So I can believe that Mandark's parents were really hippies who named him Susan. Maybe he gets his scientific inclinations from somewhere else in the family. As for retconning how he and Dexter met, well, that I will grant you, but then, there's probably a good explanation for that.
And the complaint about getting rid of the Justice Friends and Dial M for Monkey segments is pretty much ignoring the show's history considering that in the second season, there were only 2 Justice Friends segments and 1 Dial M for Monkey segment. They were pretty much on the chopping block anyway, in other words.
As for terrible pacing, Flanderization, etc. ... Well, again, I don't think these were that bad. Beauty and ugly is in the eye of the beholder.
So, in summation - the hate for these later seasons is overblown. They're not as good as the earlier ones, but they're not terrible either.
(I didn't cover Christine Cavanaugh being replaced by Candi Milo because they did get her back, but unfortunately, she retired and had to be replaced.)