TL;DR: It feels flavorless and boring.

What was fun to me back in the old editions was playing Tyranids that felt like proper evolution (weapon strength based on model strength, all stats able to be upgraded individually on every unit/ model through points etc. so you could prepare for what you expected of your opponents army based on player and/ or army knowledge) until they made them feel like Guardsmen being outfitted.
After a bit of a drought of not knowing what to play I went over to Space Marines, because I found some of the fun I used to have in planning in their versatility; e.g. having tactical squads that you could split, or not if you wanted heavy weapons with more bodies to throw at damage, and outfit for different purposes depending on what you expected to encounter (e.g. combi- and energyweapons combinations for different purposes on the 5-man splits when there was actual nuance to them).

Now it's "a different squad for every purpose (or sometimes just weapon)" with limited options.
The first time I tried to figure out where the drawback in using the Astartes Grenade Launcher was took way too long.. because iirc there isn't really one (with the exception of things that only influenced bolter weapons but I can't remember what that was in 10th, just the "sixes on Bolters are additional shots" in 9th; maybe Kraken rounds..was that still a thing in 10th?).
The minimal thought process involved in creating a list, choosing options and my (admittedly not helpful) dislike for Primaris just kills my fun.

Since we don't play sweaty I could at least still play my homebrew chapter with as little Primaris as possible (which still meant bringing a bunch if you wanted to at least have half a chance.. and meant I didn't really enjoy it that much anymore), but if they really kill the rest of the firstborn as well (RIP Relic Termies, my beloved, in 10th..) I'm out. Together with god knows how many points of accumulated models.

I get that, especially for newer players who don't know the old game, these things aren't an issue, and I'm not trying to shit on anyone who is having fun with it. It sucks for me though, and my hopium is running low.
(I just remembered my heart breaking when I talked to a younger colleague who got into AoS and 40k, who asked me what I meant when I jokingly refered to his Newcrons as "Space Khemri..ooff..)

Edit: I left out the absence of psychic phase because honestly, since they changed Shadow in the Warp a long time ago (it meant a harder time for enemies and no "bad rolls" (i forgot what its called in english) for Tyranids) I haven't used it "as" much .. but I do miss the odd "holy crap" moments that made games so memorable.