First, the effects are probably the worst out of all the films. Where TPM made a good 90’s era effort to truly integrate CGI with the shot in a seamless manner, by the time of ROTS it’s just cynically mass produced. The opening battle in particular just stands out as so obviously fake that all tension is immediately sucked out of it. Most of the action scenes suffer from this problem. And for what is primarily an action movie this to me is death.
At one point in the film, order 66 I think, a hovertank explodes with the exact same intensity as one in the ancient Battlefield 1942 video game, merely jumping into the air with magic and turning completely black. This total lack of effort tars the film completely from a visual standpoint.
The battle droids change dramatically from the more menacing and realistic looking and sounding droids from the previous two films into cartoon comic relief straight out of the upcoming clone wars cgi cartoon. They have stupid accents, make “clever” comments and seem to be made of warner brothers cartoon jelly. Again this completely defuses any tension there might have been in scenes involving the droids as enemies. Suitable for an actual cartoon, sure, but not for a live action film purporting to be of the same substance as what came before.
While key story moments from the previous two prequels were enjoyable enough, think of anakin saying goodbye to Shmi, Qui Gon’s death, going off to save Shmi on Tatooine, the final love confession etc. These types of moments are utterly absent from ROTS. And because ROTS contains most of the important story points of the prequels, that’s extremely bad. A good example is Anakin killing Mace Windu and then flip flopping from “What have I done?” to “I’ll do anything you say, my master.” in the span of seconds. Anakin finally deciding to join the dark side is such an important moment for the entire saga and it is done so poorly it would be better if it hadn’t been done at all. This is true for nearly all the important moments in the film.
Killing off Dooku right at the start of the film was a mistake since he’s probably one of the most interesting villains in star wars. His role as a double agent outwardly working against the sith but secretly using the separatists as scapegoats is criminally underutilized. Grievous is no substitute, and is almost as silly as his new cartoon droid minions.
The final fight between Anakin and Obi-Wan is so hilariously overdone it almost completely undermines the seriousness of the fight for the story. It has a few nice moments like the iconic clash from the cover art, but overall it drags on for so long you tire of it and nearly forget what’s at stake.
The film’s only real high points in my mind is the ending sequence that ties the knot with the OT into a neat little bow, and Lucas’ affinity for composing a story visually really comes into its own here. As well as the soundtrack which is probably Williams’ best work to date. And in his own words is “endlessly compelling”.
There’s too much to complain about really. So the tl;dr is “everything”.