1. The dark "entity" that Allura picks up. Wow. It's been a long time since I've seen a subplot that received so much airtime and demanded so much dialogue from so many different characters go absolutely nowhere quite like that. I don't know if that's bad writing or an homage to Xenosaga. Badum-tish!
2. The escalation of the "stakes" to Gurren Lagann levels. THE FATE OF NOT ONLY THIS UNIVERSE BUT ALL UNIVERSES OMFG riding on the final battle just doesn't fit the show. It ended up kind of losing me emotionally because things got so over-the-top with Honerva nuking every reality down to the very last branch that I knew they were going to walk it back with Nia...I mean, Space Zelda having to sacrifice herself to undo it all. And quite honestly, if you had told me at the outset that this plucky mecha cartoon show was going to come down to the heroes giving some Japanese RPG "power of humanity" speech to the main villain to get her to resurrect the Multiverse, I wouldn't have believed you, but um, here we are.
3. Quintessence. Nothing much to say here. It was a boring plot device for the last 7 seasons and it's a boring plot device for the 8th.
4. None of the themes really get a lot of weight from the writing. While it's obvious that "family" was the theme they wanted to weave through this season, with Allura and Honerva each starting it off by pining for the families they've lost, I don't think it was explored enough to make that ending resonate. I think it's just a consequence of having to wrap up the show and not finding the time, but the last episode feels like it was written with a more consistent season in mind (this is especially noticeable when the Paladins start giving that Japanese RPG "power of humanity" speech to Honerva and when Allura speaks of Lotor with actual regard).
Lastly, and this is the thing I have to rant about:
5. Crummy, lame fight scenes with no satisfying highs and far too many lows.
This is a two-part problem so I'll try to break it down. First, the crumminess: here I'm very disappointed because I know what Joaquim dos Santos is capable of - and I'll note that the non-mech fight scenes throughout the show bear his mark and are very, very good - but when it comes to the mech battles, the most fillery Gundam or Macross episodes still embarrass the final fights in Voltron. That Volt-las vs. Honervacline fight should have been the series' answer to the hallway fight from Daredevil. Instead it's basically 70% chase, 30% fight.
Then there's the highs/lows problem: namely, Voltron itself does a very fine job of getting its butt kicked all season, even after its biggest and coolest transformation, and then the fate of the universe comes down not to the best mecha-to-mecha fight in the show, but to a cosmic shoving match. You would think that after eight seasons of watching them grow as a team, the AUDIENCE would feel that special "here we go!" when the Paladins of Voltron show up to save the day, but I was just waiting to see how easily they'd end up on their backs, weakly croaking out "we can't give up!" this time. I can't root for the most well-equipped punching bags in the universe when they just keep finding ways to get outclassed by the bad guys every single time.
I guess why this last part bothers me is that a big part of the appeal of mecha as a genre is the spectacle. While you don't always need to have slick, world-class animation to make a good mecha series, you DO need to convey a sense of power, weight, and majesty to the actions and movements of the mecha. The mecha also needs to be used (and written) like a MECHA. In the end, Voltron was written like a spaceship too many times. Only when it fused with Atlas did I ever get that tingle of awe that I do with a Gundam or an Eva.