Wow, where to begin?
To put it first - i really liked it but i can also understand if people didn't like it our outright hate it ( in which case check your priorities in life if an entertainment product can get you to feel something that intense as hate, just my personal opinion).
I think the main divisive issue is the resolution via Fantasy or Sci Fi and the Duffer Brothers chose Sci Fi over Fantasy, which is a weird choice given that a fantasy game like D&D is so important in the show. Personally i expected, maybe even wanted, a more fantasy based resolution to all the questions however that may have looked like ( i'm no writer by far) but i am ok that they went with the more scientific approach to resolve it.
Given that the Duffer Brothers never tired to state that all questions will be answered ( or most? I forget) they did not lie, even if some don't like the answers. At some point early in the episode a thought popped up in my mind when i remembered the Mindflayer from early in the show, i.e. the end of season 2 and all throughout season 3 and who was never mentioned and talked about. If the DB's gotten a loose end from season 2 in the form of Kali back into the game what about the Mindflayer and lo and behold, there it is!
Thinking back on what we knew up until then it seemed pretty clear that something happened TO Vecna, that he didn't change all on his own and that something affected that change so who or what was the source of the change? Turns out it was the Mindflayer and the show was written wit it in mind and internally it makes sense so i'm fully ok with it.
That whole final fight was bonkers to the max and i couldn't help but think of how it was exactly like an RPG videogame that i love to play where you start of with level 1 characters, who can't do shit usually but over the course of the game you level them up, unlock their own backstories and the main story until it's time to prepare for the big final fight that often enough also features a twist. This final fight felt a lot like that and i liked it.
It may be strange ( no pun intended) that the characters were able to fight off such a huge monster with a simple assault rifle, a flame thrower and some Molotov cocktails when this seems to demand at least a platoon if not a whole company of main battle tanks with artillery and attack plane/helicopter support but this is not a warporn show so our heroes were enough.
As intense as the fight was ( and boy do the DB's know how to set up shots - the one where El leaps up into the air straight at the maw of the Mindflayer might replace my phone screen background picture from Eddie playing Metallic

it was also over rather quickly, maybe even too quick. Then again why draw that out into an hour long CGI slugfest?
Loved how everyone who has been directly harmed by Vecna got to watch Joyce pull a Thor and chop Vecna's head off, that must have felt catharthic to the max for them and a good way to show it really was over with him.
I expected a bit more from Kali's storyline to be honest, maybe even a side switch revealing she turned evil a long time ago or soemthing to that effect so her death came too sudden but i figure the DB's have only needed her as the story device to plant the idea of suicide into El's head and once that was done she had no other purpose and given she had the least fan score so to speak it was a rather easy decision to kill her off.
El's death however - ob boy, that will stir something for months or years when people talk about the show, including here. Some will hate it, some really like it ( maybe out of a weird reason because they don't like Millie Bobby Brown, the person) but it's one of the make or break moments of the finale and the show given how central she was to the story.
Personally i liked it even if i may have preferred a full Hollywood style ending with everybody making it and getting their happy end but i am also completely ok for consequences and some sadness and that death made that possible. I also fully understand if people get all up in arms about the "cop out" to give an alternate version of it where she miraculously survived but i liked that too because it gives both camps the option to choose their true ending, whatever they feel they may need ( same thing recently happened with the videogame Clair Obscur: Expedition 33, that has swept the video game awards this year and also features a controversial end).
I absolutely loved that the show really took its time to show how everybody was after, how their lives are going to be and how they dealt with loss and grief. The final D&D game brought me close to tears, especially when Mike narrated the alternative end for the "Mage" and when the next generation of players couldn't wait for the old generation to clear the table so they could start playing

So all considered i really liked this finale. Is it a 10/10? No but it could never be due to the hype it generated over the years. Personally i see it somewhere in the 8ish region with aspirations to climb into the 9 but not quite making it, which is a hell of a thing in itself because in recent years they were quite a few shows who have really dropped the ball when they ended so this is as good as it possibly got i think.
Last point of this rant - the possible spinoff that is talked about.
I think it may be a safe bet that a prequel might be it explaining what the deal was with that stone in the suitcase of the guy that apparently gave Henry his powers. How did the guy came into position to get it and he seemed to be terrified of it? This could be an exploration of how the military and/or Dr. Brenner learned about the Abyss in the first place and what they wanted to do with it.
So if you actually took the time to read my rant thank you, if not i understand

