My biggest issue with the Sequel Trilogy is that the villains aren't compelling.
The strength of Star Wars isn't from the heroes, but its villains. A large theme in the franchise is the idea of redemption in the name of what's right, and so you need to have a villain you genuinely care about; both the fallible human aspect, but also the credible evil-doer who really could end the entire universe.
Obviously, the best example is Vader. He got two movie series' to tell his entire arc, and thus the morals surrounding his story are some of the most core ones to Star Wars as a whole. The Original Trilogy introduced us to this horrifying, twisted, terrifying figure with a distinct breathing pattern designed to send shivers up your spine; we feared the monster. Slowly though, things become unraveled and we start to see the human side come out, and now not only is Vader an epic badass, but we really start to care about him coming back to do the right thing.
Palpatine is my personal favourite, but Count Dooku also deserves a lot of respect. Ian McDiarmid and Christopher Lee just did brilliant jobs (if occasionally going a bit hammy, but that's the territory) of building these characters who seem like respectable father figures with nice sides, and as their evil plot comes unveiled, you still are left hoping for redemption before the bitter end.
But come to the Sequel Trilogy, and who do we have that matches this archetype? It can't be the Vader-wannabe; despite the actor's best efforts, he is written as a whiny emo brat, with a cancerous addiction to a villain who has become a lot less cool after seeing his obsession. You have a masked figure (big disgusting not-quite-alien face), but for now all we know is that he's Episode V Sidious, Lite Edition, as they've refused to show us that his character runs deeper. Finally, Phasma has unfortunately become the evil side's butt monkey, and doesn't appear to have any Sith potential anyway.
Maybe I stand alone on this; I just need that contrast between the optimistic and energetic youth of the typical Light Side heroes (a lot of Star Wars stories focus on young Jedi being trained and overcoming difficult personal battles) and the cynical and standoffish maturity of well written Sith characters (a lot of whom are older, well studied and have clearly become disillusioned with the aloofness and often fecklessness of the Jedi). I don't need two sides of complete nimrods being bland.