I was going to raise the seat, but after reading this, I don't think I will... I am 6' tall and the bike feels really low. My knees bend a good bit when riding, and when I stop at a light I'm bending my knees as well.
If that's the seat gap you guys are complaining about, it's hardly noticeable to me anymore. I thought you were talking about a gap between the tank and the seat. Just paint the underside of the seat black and you'll never really notice the gap after a while. As for it being too loose, like arkkornkid described, I added some padding to make the front tab tighter and cranked down on the bolt for the rear bracket. No movement at all anymore.
But there was up and down movement on the front of the seat? Also Jon you said that you didnt have a gap between the front of the tank and the seat either so I am wondering why I have a gap there. its not much maybe less than a quarter of an inch