Jar Jar Binks, of course, was just annoying. Don't know what Lucas was smoking when he thought up that elongated waste of special effects money. The whole fight between the Gungans and those ridiculous looking robots, sort of a cross between a dog and an anorexic Daffy Duck, was just a rehash of the fight between the Ewoks and the Imperial Stormtroopers in ROTJ, yet not as much fun (assuming you enjoyed the earlier brawl).
Darth Maul might have been silly looking, but killing him off after all the hype they put into him was kind of stupid.
Pernilla August, who played Anakin's mom, looked more like the mother of Natalie Portman. Miss Pernilla probably had more chemistry with Liam Neeson than Natalie Portman had with Hayden Christensen in the follow ups.
I had nothing against Samuel L. Jackson as Mace Windu. Didn't have anything against it then, don't have anything against it now. I just can't find the hate for Jackson's presence, which I've seen in other corners of the net.
Jake Lloyd... meh. No fun watching that kid, though he seemed more into it than his successor Hayden Christensen. Kind of insulting how he just blew up that giant ship that looked like a donut with a hole in the middle by accident. Those star fighters weren't much to write home about either.
Ian McDiarmid seemed more restrained in his brief Sidious moments than he did in ROTJ where he seemed to be going for the highest ham factor possible. Unfortunately, that did not help the film (I still like Clive Revill better).
Looking back at it, I think Natalie Portman might have shown more feeling in her non-queen moments of Episode I than she did over the entire course of Episodes II and III.