Saturday Morning cartoons are pretty much the reason I got into anime
My best friend and I, and later my sister as well, would pretty much watch the lot of them religiously. Pokemon/Yugioh/Megaman NT Warrior/Sailor Moon/Tokyo Mew Mew.
That said, I don't think there's anything inherently wrong with the abstract style of modern cartoons in Adventure Time et al. Even though I don't personally care for it, I think scenarios and worlds like the ones Adventure Time establishes are far more creative than the ones represented in 90's cartoons.
Contrast, for example, Ed Edd n Eddy's small town dystopia to the setting of a recovering post-apoc imaginary continent and associated kingdoms in Adventure Time. I won't say that when properly explored a small town can't be just as creative or powerful as the Land of Ooo, but in the given example I don't think it was. Most of Ed Edd n Eddy's scenarios involved the boys getting into trouble or trying to scheme for personal gain, often borrowing on common social stereotypes to create conflict. On the other hand I've only seen a handful of episodes from Adventure Time, but from what I've seen their focus seems to be helping others maintain order via conflicts in the varied domains.
To me, one of those requires just a little bit more thought than the other, and in the example it sets for its young and impressionable viewers, I think it shows.
My bias might be at play here, since I never did care for Ed^3. But I think the nostalgia glasses are strong on the reverse side too.