He's a pacifist.
A scientist first and an Avenger second. He wants to use his scientific know-how to rehabilitate villains, but is constantly forced to resort to violence.
Basically, he's a pretty interesting character.
Also, from what I've heard, the test footage looked brilliant. When he shrinks, he retains the strength of his full-grown self, so he can be tiny and punch a guy just as hard as a full-grown man.
Hm, he does sound really different but still not starring material. I get the pacifism and constantly having to resort to violence, but that doesn't make for a starring hero for me. It feels like it would be a lot of:
Ant-Man: You don't want to do this!
Big Bad: YES I DO.
Ant-Man: Well, okay. *punches him, saves the day*
I just don't get the big name appeal of Ant-Man. I can see him being a great character, but not for his own movie. He should have just been introduced in Avengers 2, maybe had some scenes where he plays a big role but I just don't understand the grandeur appeal of Ant-Man as a big name hero.
Edited:
Well, he can also turn huge.
So can Captain America, judging by this poster: