I feel the same way. I like Elmo and Abby, myself... it's just hard to find a project to like them in. Elmo doesn't emote when all he does is talk to the audience about Bowling Balls for 15 minutes. I say, if you want to see the real Elmo check out something like Elmo Saves Christmas and Elmo in Grouchland. We can see that he's a child, and dispite the fact he is willing to share, a lot of the times, he's quite selfish... like not noticing how his wish of Christmas everyday ruins lives until its too late...or seeing his own selfishness in a funhouse mirror of himself, trying to get his blankie back.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).
I see the same thing in Abby too... we know she idolizes her mother, she's proud of her fairy culture... but at the same time, when she screws up a spell she either reacts "oops! Sorry! I'm still learning" or "I'm ashamed and depressed by my mistake." She also has trouble distinguishing between her faiy tale world and the "reality" of Sesame Street (obsessively trying to kiss Oscar because she thinks he's a cursed prince), as well as her rush to grow up seen in Abby in Wonderland.
When Abby first appeared, the writers clearly had trouble of how to establish her and make her a star at the same time. Every other thing coming out of her mouth seemed like a catchphrase brainstorming session. I think the writers have established her enough, but they're still teetering over how to use her.