Episodes like Lisa the Vegetarian, Mother Simpson, Marge Be Not Proud, Two Bad Neighbors, Lisa the Iconoclast, Curse of the Flying Hellfish, Much Apu about Nothing and Summer of 4 foot 2 were all episodes that felt very different from what had come before, and in not-so-great ways. You get more topical and preachy stuff, more seriousness, the emergence of "Homer the annoyingly unfunny do-gooder" (with the Apu episode). The sideshow Bob episode felt forced. Some great lines or whole scenes in some of these episodes, but the feel was different and at the time it was impossible to not think "this is not what it used to be."
Still some great episodes, though - the Treehouse installment was awesome, IMO, and also Radioactive Man and King Size Homer are favorites of mine.