Those who loved this film...

Memories of my youth.
Cliff seemed like a real man, a traditional man. He did what needed to be done, and lived with the consequences of some of his choices.
How Cliff and Rick treated each other with respect. There was no envy or jealousy.
The western themes.
The movie /tv show making.
Watching Rick overcome his difficulties.
All the details only the knowledgeable would appreciate.
The joyful feeling it had, while at the same time having a dark cloud come in every once in a while to remind us of the history.
The acting.
The cars and the driving.
The cathartic ending.

And from a believer's point of view, this:
https://www.thegospelcoalition.org/article/upon-time-power-movies/

I think it might be a generational thing. I am just about the same age as Tarantino and this I completely relate to. I loved the entire feel of the movie. I felt as if I was transported back to my childhood. There were so many references packed into this movie that it felt almost as if the movie were actually made in 1969. As a young child television and radio made a deep impression on my mind and all of that came flooding back to me while watching this movie. There were so many instances where I had the same feeling for something that I had way back in 69 and not all of them good. For instance, 1969 was a very turbulent year and even the young child in me could feel it. There is one scene where Cliff and Rick are driving up to his house and there is a spooky ad for Ray Bradbury Theater playing on the radio. Now, being so close to the Sharon Tate house also gave that scene an extra creepy feeling, like, OK, here we go. We're now heading into the mouth of the beast. Through the entire movie people who know about Manson and the murders, probably were feeling that same sense of dread for what we thought was to come. Fortunately, that didn't happen and it was a welcome relief. That ending was so unbelievably cathartic for me and probably many others. I liked that we saw the Manson Family members getting their asses handed to them by Cliff. It was really awesome.

The acting was so good also, especially by Brad Pitt. He was so in the role. No matter what was going on, Pitt was there. Whether it was killing or beating hippies, kicking Bruce Lee's ass or just simply fixing the TV antenna; Pitt was riveting.

I could go on more, but I probably suspect that you didn't like it, so much that I say may not resonate with you. I loved the movie. It was so different from anything Tarantino has done in the past.

Bruce Lee getting launched into the side of a car

Rick Dalton’s journey did it for me. I am a fan of B films and especially Italian B films. I’ve seen so many of the same American actors in the 70’s appearing in those productions in many cases probably for the same reasons Dalton did.

His melt down in the trailer about screwing up his lines and the drinking was captured damn perfectly, at least I can say the drinking part was. The scene where he gives a great performance and the opinion of the little girl means the world to him is the first time ever in a QT film that I have personally felt emotional.

The entire dynamic between Dalton and Booth is just amazing to watch. I really feel like QT branched out into new territory with this one. I was born after 69’ but with a good grasp of history and film I feel like I picked up on all the little references of the time.

I can honestly say I loved every part of this film and it’s hard for me to relate to those who say it’s boring.

So many things... it's one of Tarantino's best, in my opinion. Among the things I enjoyed:

1. The depth of the plot
2. On a subtle level it wasn't just nostalgic in that it recreated 1969 LA, it recreated movies as they were made in 1969.
3. The humor
4. The Bruce Lee scene!
5. Brad Pitt absolutely nailing the part of a John Wayne archetype come to life.
6. Leonardo DiCapri equalling Pitt in what may have been the more challenging role.
7. All the subtle touches, things that only come to light upon reflection/ repeat viewing.
8. Tarantino gave us his first truly happy ending.
9. The entire Spahn Ranch sequence; so eerie, so incredible
10. The climactic fight, and the way Tarantino is able to show action onscreen without resorting to useless cuts or shaking a camera and instead presents battle in such a way that the audience always knows where each participant is, and the fight flows realistically and understandably.
11. Even though the film had a languid pace, I always felt a sense of forward motion. I was unnerved and on the edge of my seat during my entire first viewing, never quite able to feel get my bearings and with no idea at all what was coming next. This is a very, very rare quality in a film, and one I deeply appreciated.

I went into a lot more detail about most of the above in a post I wrote here:

https://moviechat.org/tt7131622/Once-Upon-a-Time-in-Hollywood/5d4073e80b25b80834d1343f/Comparing-Booth-and-Dalton-Spoilers

The laid back vibe, that you were just going along with these people as they hung out in LA. Unusual for a modern American movie.

I really like the Mod style, the big American gas guzzling cars, the household decor, the music...I swear my mom had half of that stuff in the house growing up

The scene at the Spahn Ranch where Booth meets an old pal and lays down the law for the psychos...

Dalton's several breakdowns while playing an on-screen tough guy...I loved the scene with Leo and that bright little kid, Leo is the gift that keeps on giving

The finale which I wont spoil but was so outrageously wild that I think I laughed throughout except I may have gasped at times!

This one is one of Tarantino's very best
movies

I liked all of the 'driving around' scenes too
This movie was pretty much perfect

So it seems like what people like most about this movie was the nostalgia?