I've started a new Skyrim game about 3-4 times but quit each one at around 20 hours because I wasn't satisfied with my character.
My question is should I wait for Dawnguard before starting a new game? I'm planning to get 100+ hours on this save so I want to start it at the right time.
Dawnguard information will be released at E3 which is in less than 2 weeks from now. If you are not sure whether you want to start a character, I recommend two things:
1) Play through the beginning one or more times. You have mentioned that you've started and stopped multiple times because you were not satisfied with your character. Chances are this could happen again. So keep trying until you are satisfied with your character.
2) After E3, you will have a better idea of what the DLC entails. At that point, based on the content, you can decide whether you would rather have it be in the game from the beginning or whether you would rather start playing beforehand.
Personally, I'm mostly playing Diablo 3 now, and after I get tired of that, I'm going to start a long term character. I figure that, whatever happens, I'll probably be playing him when Dawnguard comes out.