I haven't run it yet, so my feelings are preliminary. As a DM who has a 12th-level twilight cleric in his current party of six PCs, I chuckle a little at the idea that character buffs might be a problem.
My strongest impressions are that they had a much stronger grasp of the action economy and resting mechanics this time around, and I think that's a fairly big deal. I'm seeing many fewer instances of "no one will spend an action to do that" abilities, as well as bonus action issues, which plagued PHB 2014, SCAG, and even Xanathar's before they started to figure it out. And I like that they seem to have given most classes a reason to care about short rests, apart from hit point recovery. Going ahead and doing this for all classes would have been a nice touch... Still, I think these changes will make the game run more smoothly at the table. Nova damage seems to have taken a nerf and most classes get some tools to extend the adventuring day.
Other than that, feats and spells overall seem better designed. I think I'll really like the effect the new Great Weapon Master has on the game (along with the corresponding Sharpshooter nerf). I love that it applies to longbows and heavy crossbows. It looks too strong, but at least it's the right sort of thing to be strong (for my tastes), as opposed to hand crossbow cheese.
I'm not sure how I feel about the healing buff (this is actually related to the action economy impression). On one hand, it should definitely make in-combat healing more tempting. On the other hand, I'm not sure that's a good thing. Whack-a-mole was a dumb artifact of 5e design that made combat far more cartoonish than I want in my games, but I was able to house rule that away. The fact that the game didn't need "healers" to be using their actions in combat to heal was a plus, to me. I think it will probably still be fine, whack-a-mole will still be a thing, and I'll still use my house rule for 0 hit points. But we'll see.
Preliminary opinion: It's a better designed 5e. I'm not sure the design is as tight as it should be given ten years with the system, but it's still better.