Do I like this Avatar guy who now stars as Perseus? Not especially. He's just really... meh. Anyway...
Things I liked:
Ralph Fiennes as Hades. I loved how he played a similar character to evil Lord Voldemort, yet he played them so differently. It reminded me of how I played two nuns in two different productions, yet I was able to keep it fresh. ;) (That's me comparing myself to Ralph as a joke.) Have you seen his portrait on imdb? He kind of looks like he could be an evil lord: http://www.imdb.com/name/nm0000146/
I liked all the action business going on. Fun times with giant scorpions!
I thought Medusa was freaky. Too much laughing.
I like how they replaced Bobo the Owl with a Bond girl. That was probably smart.
There are obviously things I don't like. It's not the best movie ever, but it's fun, teaches you a bit mythology, and is what many other movies are not: entertaining.