Lucas has said "the point of this movie is to see how a good person turns bad. Thats why i set Episode I up to introduce Anakin as an innocent--not 'well, he was a demon-seed, so it was obvious that he was going to turn bad.' But to explore how people turn to evil and the choices they make."
Well, here is the thing--this, the most crucial linchpin in Anakin's character--is 100% absent from the film. It occurs in Episode 1.5.
We meet Anakin as a 9 year old--cheery, full of compassion, resourceful, always helping people, the sweetest kid you could meet. What a brilliant irony to present Vader this way. Then next we see him--he is angst-ridden, emotionally-upset, power-hungry and with aspirations of dictatorship. What the fuck? How did we get there? Who knows! Use your imagination--assume things. Asssume everything. The problem is that these issues are the core of the PT, and according to Lucas they are both the main reason why Lucas wanted to do the PT and the only reason Lucas introduced Anakin as a child in part 1. But he not only botched it--he completely threw it away. He didn't even try. The Anakin in Episode II is not the same character from the previous film--i never even once imagined that Jake Lloyd grew up to be this person. And whatever reasons that made him become that way are not even so much as hinted at--its not that we have to use our imagination: we don't even have a frame of reference.
Thus, we don't understand why he's a whiney brat, why he's a power-hungry prick that can't take hearing the word "no." Not only do we not sympathize with him, we don't even understand him. Thats the reason why people hate him. Luke was a bit of whiner in ANH but we could understand his situation because we've all gone through that kind of stuff. But with Anakin people just say "what the hell are you whining about now? Shut the fuck up and be a man." And its because there's absolutely no character motivation. People say "oh well he's a teenager." That doesn't cut it. "Oh well he was a slave and separated from his mother and has no friends." That doesn't cut it. Show us these things--or at least indicate them. Thats the real issue--most people who like Anakin or claim to understand him are reading into his character things that aren't there, probably simply because they want to like him.
And thats one of the reasons i thought ROTS was kinda good. Finally there was character motivation and emotion. You could understand why Anakin was confused in the first half of ROTS, where everyone is telling him to stab people in the back, you could understand Anakin's fear at loosing his loved one and the drive that made him become obssessed with finding this greater power. The first half of ROTS is great because its the way an actual dramatic movie is written--ie, not a piece of shit (for the most part). Once Anakin kills the kids of course everything falls apart because the arc gets destroyed (due to the revisions as stated before).
I also came to another conclusion today--in retrospect, many feel that TPM is the closest to having the "Star Wars feel", even though the style and aesthetic are completely dissimilar to the OT. But, I finally realised why this, the movie that should be the complete opposite of the OT (and is, visually) still retains a strong Star Wars feel through much of it. As CO pointed out, the OT is a macro story. And so is TPM. It has no central character, although it revolves around Qui Gon Jinn--just as the OT had no central character, though revolved around Luke. Jar Jar and Obi Wan share equal screentime with Anakin, just as the droids and Chewie share equal screentime with Han, while Padme acts as a plot motivator just as Leia does, with Qui Gon centring the film just as Luke. Theres also a clear and very active villain, unlike AOTC, with a somewhat lighthearted plot and scenes that at least develop and resolve instead of being rushed through like the sequels.