Kate, I'd been meaning to watch the episode again and give you a decent reply, but I can't track it down here - it hasn't yet been released on BluRay in Australia, and I have to go from memory (at least two years since I saw it).
Anyway, we agree the scenery is terrific, I also think the obelisk is one of the best set construction efforts (apparently they lost control of costs while building it, and finishing it strained the paltry third season budget even further).
The 'Preservers' concept is an intriguing one, and makes a lot of sense. If we have the ability to move hundreds of people across space, it's inevitable someone will use that power to intervene and 'rescue' cultures by relocating them.
This episode is unique in that it takes place over months - Kirk had plenty of time to mourn his losses when the Enterprise limps off for the long trip to get repairs. And, besides, did we ever see Kirk in his quarters mourning Edith Keeler? It doesn't seem to take the shine off City, that its events were never followed up.
Shatner's acting : well, hugging himself to show us how contented he is living with the Indians isn't one of his best acting choices, to be fair.
Arguably, this episode features the best special effects work of the series (I'm talking the original craftspeople, not whatever the CGI team did).
Unfortunately, I have little knowledge of Native American culture, or how well the episode represents it, may be it is a completely ridiculous, cliche-riddled depiction, but I wouldn't know - on the other side of the world it looks fine to me.
I don't mean to eleveate this episode to the top shelf of TOS viewing, but I think it is generally underrated around here, and a stand out in a massively below-par season.
The Native American elements are, of course ridiculous - for one thing, out here in Reality Land there is no such thing as
a Native American cultur
e; there are instead many separate and distinct cultur
es. Here it looks as though the writers took their dinner plates down the Indigenous Peoples of North America Buffet and selected a few bits here and there, as it struck their fancy, with no consideration about how well those bits fit together, fit the landscape, or fit the costumes (which they appear to have borrowed from the props department of a Western that favored Generic Plains Indians as its native people of choice). I hate that. But I talked about this earlier in the thread so I'll try not to berate the point. Too much, anyway.
I am also a bit puzzled by your admiration for the concept of the Preservers. Surely we've seen this concept a LOT before, even on Trek? The show is full of mysterious alien objects/computers/systems/thingies that take care of races that apparently can't be trusted to take care if themselves: Landru in "The Return of the Archons" (cue spooky computer voice: "You will be absorbed") and the androids in "I, Mudd." Ooh, and even closer parallel that I just thought of is the God/Computer thing in "The Apple" - I can't remember its name right now. That episode is also notable as the winner of Worst Wigs in Trekdom Award, BTW.
In fact, right there in season three, though I admit this is after "The Paradise Syndrome," we have the ship in "For the World is Hollow and I Have Touched the Sky" and the supercomputer in "Spock's Brain."
So the whole Preserver concept, or so it seems to me, was already more than a bit overdone when the episode first ran, and it hasn't improved with age.
I'd just like to point out that most of the strengths that you mention are all...well, superficial. How a episode looks is a factor in its quality, but at least for me, its importance is well below that of the writing and the acting. And here, neither one is very good, IMO. Well, OK, some of the acting is fine, but Shatner's is not, and since he's the star, this is a big problem for me.
But hey, if you enjoy it, more power to you. We just started watching all of TOS in order, so I think - unless I'm not feeling strong enough that day - that I'm going to watch it when it comes along in the cue. I don't expect to enjoy it, but I could be wrong. If so, I'll let you know.