Sure, there's bad dialogue. Anakin's attempts to be poetic are painful (though, realistically, when would he have cultivated a poet's gift with language?). Yet is it really any worse than most of his dialogue from The Phantom Menace? Sure, there are nonsensical plot points. (How easily Amidala forgives Anakin for mass murder!) But are they any worse than the unjustified story turns of The Phantom Menace? No, I have to say there aren't many places where Attack of the Clones is truly bad. I'd say its larger problem is that it's boring.
The stakes laid out in the plot are quite abstract. The Republic is on the verge of collapse, but it's a dry, political problem until the final act, and difficult to relate to. Personal stakes would help immensely, but the only character who has a real arc in the film is Anakin. Obi-Wan spends the movie alone in a film noir mystery. And Amidala is only there to fall in love (unconvincingly) with Anakin. Certainly there's no growth or change for Yoda, Mace Windu, the droids... or anyone else.
Accentuating the boredom, there really aren't that many action sequences in the movie until the final act. And nearly all the action sequences that are in the movie don't feel realistic enough to be engaging. The car chase on Coruscant feels like a drawn out version of something that was done much better (and more succinctly) in The Fifth Element. The conveyer belt sequence feels like a video game level that you don't actually get to play. The arena fight is so much lifeless CG, you could probably take the generic reaction shots of Ewan McGregor, Natalie Portman, and Hayden Christensen, and make a whole new scene in a wholly different context.
Speaking of Hayden Christensen, lots of people point to his wooden performance as the biggest fault of the film. I don't agree. I noted of The Phantom Menace that lots of fantastic actors came off flat there, thanks to George Lucas' inability (or disinterest, take your pick) in working with actors to elicit a good performance. Christensen is decent in Shattered Glass, and quite good in Life As a House. In fact, he has pretty good chemistry with Ewan McGregor in the opening act of this film -- the two have a decent banter going between them.
The problem with Anakin is that the script is trying to make a Han-Leia romance of he and Padme. They're supposed to find love amid danger and duress. But where The Empire Strikes Back built upon already likable characters established in a prior film, here we're starting with an older Anakin played by a new actor. Han's barbs thrown at Leia seem playful. Anakin's quips toward Amidala seem petulant. When Amidala confesses her love for him, the audience can only ask, "why?" -- because he's only alternated between whiny and creepy for most of the movie. (If the kiss had at least come after the meadow scene and the dinner scene, that would have helped the love story's progression -- in those moments, Anakin is at least slightly endearing.)
There are a few elements of the film that do work. Ewan McGregor really channels Alec Guinness at times, adopting the original Obi-Wan's speech patterns (though it's a shame he spends most of the movie alone, or acting opposite CG characters). Jango Fett is shown to be the badass in a fight that we were always supposed to think Boba Fett was in the original trilogy, but never actually saw. The visual design is excellent, very cleverly evoking styles that we saw in the original trilogy.
But when the mind is bored, it starts to wander.
- This Jedi Sifo-Dyas was an actual person that multiple characters in the movie met. So why does he have a name that's so close to "Darth Sidious" that you'll surely assume it was an alias?
- Shmi really held out for a month only to die 20 seconds after her rescue? Couldn't she have just been dead when he got there?
- If C-3PO worked at the Lars' moisture farm for years, shouldn't Owen recognize him when he purchases the droid from the Jawas in Star Wars?
- Given how much the fans hate Jar Jar, is it actually a good thing for the story that he enacts the vote that literally leads to the destruction of the Republic?
- Why is it they can pass a vote to empower Chancellor Palpatine to make an army when they can't pass a vote to just make the army?
- Sure, C-3PO has always been comic relief. But when did he just become a pun factory? ("What a drag." "I'm quite beside myself.")
- Is it a good idea to invite blabbermouth Threepio to your secret wedding?