I haven't read comments online so maybe already covered, but I've heard people complaining about how long the last 2 episodes were. I get that, they were basically movies, but that's not the real issue imo.
The problem was how drawn out the final battle was. Vecna bouncing back and forth between his victims to deliver long...dramatic...pause-heavy...lines may have actually worked against the intended effect.
More difficult for me to accept was just how much of a continuity or pacing discrepancy there was with various scenes. The tentacles begin choking the small strike force in the haunted house and it seems like they're about to die. Then we get what was probably 15-20 minutes of additional footage of other battles that are supposed to be taking place simultaneously. It just didn't mesh as well as it should've.
Another complaint I have is I KNEW YOU WERE GOING TO KILL OFF EDDIE, YOU f***ERS. But I guess that stays true to the 80s cliche of the metal and punk guys being expendable elements of society. Still, FU.
Also Dustin's crying during Eddie's death was unconvincing. I think that actor has struggled to find his form since the growth spurt. Same with Will.
OTOH the montage with Eddie playing Master of Puppets was f***ing aces. Perfectly done. Chills.
Lastly, what happened to the General and his pursuit of 11? Are we to believe that a downed chopper and a few blown up vehicles was enough to stop a black ops mission of the US Government? And a handful of kids conspicuously racing from the scene in a cartoonish pizza truck would't be easily spotted well before ENDING UP AT A PIZZA JOINT OF THE SAME NAME? Instead of being hunted down, 11's friends begin openly assisting in a government-managed disaster area filled with soldiers, shortly after a major incident that would likely still be blamed on 11. It didn't make any sense.
Also don't like what happened to Max but that's the way it goes, I guess. I assume she's portrayed as braindead based on 11's inability to find anything inside her mind, but we'll see. The ending sets up for a season 5 which surprised me since I was under the impression this was the final season. Guess not.
Other than those plot holes etc I thought it was good overall. Maybe I missed some things and it will make more sense when I watch again.