Small things:

In the opening scene there is a moment when Ewan McGregor makes a face like “oh, this is just soo corny”. And after that the movie was ruined. Even if I did not want it to feel corny, it did because of that scene.
Starship designs like the “Uglies” of EU: cobbled together from various parts of X-wing and TIE fighters that we are supposed to recognize from the original trilogy.
For instance, the Jedi Fighter, which has large curved windows and TIE-fighter like window, with a thick beam separating them: right in front of the pilot’s eyes.
“S-foils in attack position”: Eh, no. S-foils have guns on them. “Attack position” is when the guns are spread wide for wider dispersal of laser fire. Tiny flapping baby wings on larger wings do nothing.
“This is where the fun begins”, said non-ironically.
Buzz droids
New villain is a cartoon character
… and that’s just the first few seconds of the movie.

Big things:

Padmé is diminished from starship-piloting warrior queen in previous movies to … someone who dies from a broken heart … over a complete dick.
Anakin’s fall to the Dark Side is weird.
The Emperor’s transition to wrinkly
So many unanswered questions in the previous movies that were not told in ROTS. Instead we had to get them from novels set before and after the movie.

Of all official Star Wars movies, I rate Revenge of the Sith last.