“Everything” is the correct answer. I will break it down a bit, however:

The characters.
Terribly written and awfully acted. Mace Windu, Yoda and to a lesser extent Obi-Wan are a bunch of dull monks. Grievous is silly. Palpatine is way over the top. Anakin is less awful to watch than he was in the last movie, but his transformation is handled very poorly. Also: Darth Vader is now a child-killer, yet his mentor and his wife still kinda like the guy.
The story.
Most of the time it feels like George had a checklist of stuff that needs to happen in the PT and only got around to using it by the time he started making the third film. Having said that, the PT doesn’t even tie in to the OT very well and introduces a lot of continuity errors. I also hate how every surviving character (except for Chewbacca) ends up in the exact same place where they are found 20 years later.
The little things.
They had the same actor who portrayed Palpatine in ROTJ. They could have take advantage of that and with some very mild make-up work he could really become a 20 years younger version of the same character. Instead they chose to glue an awful Halloween mask on his face.
The two lightsaber duels lack any suspence whatsoever. I never found out why the Yoda vs. Palpatine duel was even in the film.