I'm going to keep this short because, frankly, to even review this movie is giving it too much credit.
It sucks and is awful.
Basically, if you took all the exciting parts from 300 and Percy Jackson: The Lightning Thief, and took out all traces of fun, humor, originality, or character development, you would get Clash of the Titans.
This is as movie everyone has seen before. Every scene and character design/archetype has been in a movie you've seen before. The battles are lame and repetitive, the effects (besides the Kraken and Hades' smoke) are outdated, the makeup looks more fake than anything of recent times, none of the jokes hit the mark, and there is NO CHARACTER DEVELOPMENT. FOR ANYONE.
And the worst thing of all is that it's just plain boring. I almost fell asleep on several occasions, and that has never happened to me, not even during March of the Penguins.
There really isn't anything left to discuss regarding this film. It quite sad that, for a movie with a decently exciting trailer, this didn't live up to any sort of expectation at all.