I only read a couple of reviews and they both said that the story was solid, but the character makeups, settings and CGI couldn't stand up to it.
They made it sound like the seriousness of the story was cheapened by mediocre acting and effects... which honestly sounds like "film" critics faulting something for being high fantasy yet again.
I'm not even a WoW fan and I'll probably still go see it. I'm sure it will be entertaining, at least.