I never thought the TOS was that utopian though, TNG definitely was, and sure TOS does a lot of "perfect stars, flawed guest of the week" type but TOS seemed to have a lot more "no real winners here" episodes about whoever the Enterprise visited. At least in the first season.
I guess I just liked the twist of DS9 forcing everyone to stay in place and face their consequences. Every other Trek has them in the middle of the Federation shuffling some diplomats, then the next week they're out on the edge of space with WEIRD ALIEN THING, and then they're staring down the Romulans/Klingons/Cardassians/etc. the next week.
DS9 was still pretty utopian. Garak and Quark and Nog and Odo could only knock the humans hubris so much. Even the Dominion War where billions upon billions died and tens of thousands of ships were lost only seemed like it was going on every other week.

DS9 did do a lot to damage the "all aliens of a species aren't individuals" thing though that TNG started with Worf.
EDIT: Having recently been watching the TOS remastered and TNG remastered, the latter has a very distinct lack of tit racks. TOS's kinda surprising for a 1960s show at times. Didn't get tit racks like TOS until T'Pol had to decontaminate in the first episode of Enterprise.