Killing off Eddie was stupid af (it's been 3 days - fuck spoiler tags). He's about 100x a stronger character than the California crew and could have carried a subplot by himself. Instead, we have Byers clan + Mike who were basically relegated to being Eleven's chaffeur this season because they all suck as actors. It's an unfortunate turn of events for the Duffer Brothers, who had no way of knowing back in 2015 that puberty would be so unkind to the Mike, Jonathan, and Will actors (I'm probably an army of one on this but I think a lot of the ire directed at Dustin is unjustified - much better actor & character than the other male "kids").
The Russia & hopper plotline was completely unnecessary - not sure why they just didn't let Hopper die at the end of season 3. Max should have died as well to complete her character arc.
All that said, I thoroughly enjoyed this season and thought that the flashbacks to Hawkins Lab and the Henry/Vecna/One character were among the highlights of the series. Music was as on point as always.
It'll be interesting to see how the characters have aged when the series concludes in...2024.