The story was boring and cliche. There was nothing surprising about it at all, and I've never read a Fantastic 4 comic book.
Also, the special effects were really awful. Well, mostly just Mister Fantastic. It looked exceedingly fake everytime he would strecth.
I also thought the story moved way to fast. One second they're outside a building, the next they're in space, the next they have weird powers... And the movie did a horrible job of blending jokes with drama. At the serious parts, I just wanted to laugh and at the "funny" parts, I was bored out of my mind.
I really didn't think the Human Torch was funny, even though they tried to make him the comic relief.
Bah. Horrible movie. Especially if you haven't read any of the comic books. The writing was so terrible.