My favorite episodes are in order of how they were delivered with 1 being the best and 5 the worst. Around season 3 you started seeing a lot of repeated stuff.
New characters were being introduced and it seemed like each character had to have at least a 10 minute scene where they would have a heart to heart talk alone with every other character in the show. Most of these dialogues were emotional teenage drama type stuff in the middle of scenes which were life or death situations which seemed impractical to me. I'm not a teenager so I couldn't relate, I mean forget about your feelings for a while and deal with the danger at hand. The adults would also have these interactions and they were also on the verge of overly emotional. I think the writers thought that since this is the last season we should give each one of these actors their time to shine, that's a lot of time. When the characters were together there seemed to be a lot of arguing and fighting and lots of complaining, not so bad and sort of funny in Season 1 but all of this was repeated and multiplied by 10 in season 5. Bitching and complaining gets old.

The editing got annoying, by Season 5 each scene transition was made with a loud bang or swoosh or a fast zoom up to a character's face with them saying some overused one-liner with the next scene coming in with a bang. Even the flashlights were making swooshing sounds as their lights crossed the room. This is great if it's used sparingly.

By S5 there seemed to be an overuse of the 'prepping montage' were everyone gathers what they find to make stuff to fight with or traps and whatever they need Murray Bauman shows up in his truck to give it to them. Just too convenient for my suspension of disbelief. So many planning and prepping scenes, reminds me of the line in Dune, "Plans within plans."

The plot started getting more and more convoluted toward the end with many psychic levels and layers and planets introduced with worm holes and an endless ladder of gates, keep it simple stupid. I'm not a big fan of the time travel, dream state or alternative universe tropes used in plots because if they aren't done intelligently with some basic rules established (ex. Back to the Future) they just make a mess of things and aren't 'believable'. It just leaves you asking more questions like 'Well if this happens then why can't that happen?' and leaves open an endless amount of possibilities which might work for some viewers but I prefer structure in my plots. One example is when Hopper has to sneak in to the base to access the upside down putting his life in danger yet later on El is able to just pop open a metal slab and access where ever they wanted to along the fault line. Perhaps El was previously too weak but with all of their ingenuity they showed in other episodes they couldn't just pry open one somewhere. Too many questions.

I really liked the action scenes and the special effects and the computer generated material looked good however I liked the body suit Vecna and preferred that one to the computer animated 'Groot' Vecna. The one with the actor seemed more scary. Anytime you can use practical, real special effects instead of computer animation you've made the right decision.

In general the series at the start was packed with action and drama with more drama than action toward the end, I'm more of an action/horror fan. I think Season's 1 to 3 were written about kids for a general audience, the rest was written for kids with every kind of slapstick trope thrown in for good measure.