I think the ending episodes were satisfying enough and in keeping with the ultimate overall tone of the series, despite some clumsy writing.
*I never vibed with Henry/Vecna as a villain. I don't think his Vecna appearance was weird or scary enough. They should have dialed it up like 300% on the Cthulhu meter. I thought he was much more effective (and creepier) as just a human.
*My favorite part of the show was always having legitimate (if obscure) rules to their supernatural elements that could be discovered or reasoned out by the mundane characters and exploited. From the using lights to communicate with the missing Will in season one to scenes like creating the demogorgon trap in season five. Love it! Let's A-Team this thing!
*They ended up with way too many characters. Seriously, 20 characters is a good 10 too many.
*Everyone had too much plot armor. Stephen King wouldn't have been so kind. Obviously, with the close connection between the core characters, any loss would have been devastating to the survivors, and seriously changed the tone of the series and I can understand not wanting to go there, but man...they couldn't even kill off Moms Wheeler (and she gets a post-crippling hero moment!).
*The Dr. Kay/military side of things was underdeveloped. The government seemed to have invested a lot of money, effort and lives in this project. Why exactly? To make more Elle's I guess. Why do they need more Elle's? What would they do with them? Why do they think it won't all end in disaster again? They could have easily had a scene wherein Kay explains her "vision" and what she's fighting for, but we get nothing. They also let all of the main characters go at the end, even the ones that straight-up gunned down a bunch of their fellow soldiers. That was nice of them.
*I did like the ending D&D scene. I think it's obvious that Storyteller Mike later becomes an uncredited creative consultant for TSR and WotC. That explains the D&D-anachronisms perfectly.
*I loved the D&D module art style for the ending credits. Worth the price of admission for that alone.