Don't get me wrong - it's not the worst of the PT movies - that would be TPM. No, ROTS pisses me off because this was the payoff movie. This was supposed to tie everything up beautifully. This was supposed to deliver a punch to the gut.
Where were the shocks? Where was the actual tragedy? We were shown everything and yet we understood nothing. Anakin just kinda turned bad for no real reason - his justification for turning to the darkside was Padme. Yet, in the end he tried to kill her.
The Phantom Editor was 100% correct in saying that Lucas' serious problem as a mature filmmaker is that the "good guys" are really good and the "bad guys" are really bad. If we get a young adult Anakin in Episode I, and his character is properly established as an anti-hero with a lot of grey area, his character can evolve into evil for all of Ep. I and II, rather than a quick flash of evil in the middle of Ep. II.
Lucas built many parallels from the OT into the PT - so why not establish Palpatine's mentorship of Anakin right at the start of Ep. I, as Obi-Wan was in the OT with Luke?
Dialog problems, sophmoric/unsubtle humor, and overreliance on digital effects are rife throughout the PT (amongst many other problems), but the ultimate failure of the PT is most closely tied to the inability to properly flesh out the two critical characters, Anakin and Palpatine. ROTS emphasizes that failure more than the other two PT films.