I think it has more to do with the characters as characters...While Grover and Forgetful were more well rounded, Elmo and Abby are more happy go lucky all the time (that's not to say that they don't get sad sometimes but still) and sometimes that can really get to a certain group of viewers, parents, and even the target age group, making them not care for the characters as much or think SS is 'for babies' since they in a large way could be held as symbols of taking the show down to the more simple level it is now at, which could at times be seen as talking down to the average preschooler (but then a lot of shows now days do not give younger children enough credit for what they might be able to understand and not get upset about all on their lonesome).
Some of the hate as far as I understand it, could also be linked to the fact that they have been really pushing Abby lately and the idea of her own show could be seen a bit as overkill. I think the fact that (unlike when Forgetful got a series) the street scenes are getting shorter and shorter and the character segments are now much longer (I think) could be doing a lot to support this bitterness, as it can mean in the long run less focus on all the other characters to fit in a theme song
.
I'm not saying I agree or dis-agree with these feelings, only that I can understand why someone could feel that way (even little ones) and that I think that it is heathy to talk about what makes us unhappy as well as happy as long as we never give up hope on SS. That is what is most important IMHO