I hate on him for a variety of reasons.
1. Horrible decisions. He's a total brat with no regard for anybody but himself. I don't feel like going into detail on the individual ones right now. He was at least heroic back in the beginning, but that ship has long sailed. Also, he's a total tool, doing whatever people want of him under the guise that he's using them.
2. Horrid personality and character development. Yes he had a horrible childhood occurence, but everything since then he's brought on himself. He utterly rejects anybody who tries to reach out to him (team 7, Karin), and even tries to kill them. Compared to other characters, especially his own brother, his background really isn't that bad. In the first series he was somewhat likeable if you ignore the emo brat aspects, but after leaving to Orochimaru he had no personality whatsoever. Kakashi was almost able to talk sense into him, but the sound 4 took advantage of his lust for power forever screwing him over. Honestly, what did the guy expect to do after beating Itachi? He did have an oppurtunity to come back a redeemed hero after the fight, but he wasted that. When he finally did show personality, he was a crazed idiot. He has both a superiority and inferiority complex. He's attacked much stronger characters than himself with little to no prior planning expecting an easy win, including all 5 kage. Predictably he got thrashed. The whole vengeance against Konoha who only ever did good things for him made no sense whatsoever. If you're going to make a 'cool guy' type of character, don't make them a misguided selfish emo brat. Take Minato Kakashi and Itachi for example if you want 'cool guy' done properly.
3. Highly overrated. Let's face it. What fights has he legitimately won without help? Sure he has great powers, but even with them he still gets his head handed to him, and spams to the point where he's gotten sloppy with his regular ninja skills. Susanoo vs Zetsus? Seriously? The best you can really say is Deidara, who was fighting with type disadvantage and still pulled to a draw only to be saved by Kishi's divine intervention. Speaking of which...
4. Kishi plays favorites with him way too much, often pulling random stuff out of nowhere. Let's see, magically knowing how to counter Oro's possession, summoning manda hypnotizing him and jumping in his mouth in a fraction of a second while out of chakra, Juugo trusting him instead of murdering him, surviving quite a few attacks that nobody has business surviving (susanoo makes for a lousy airbag), random sharingan upgrades on demand, and somehow knowing how to revive orochimaru.