While there is no doubt that there is a bit of a talent difference at each level, you can still find D1 talent at D2 and D3 schools. I have to say the biggest difference I see between the divisions is consistency of the players. A lot of the big time players at big time schools perform day in and day out and go about their playing in a very business like matter. Not saying that some players in D2 and D3 schools don't play like that, it's just not as common. As far as velocity, I don't think it's different at each level at all. For example, there's a kid on my team (Newberry College Div II) who is consistenly 94-96mph. He doesn't really have great secondary stuff, but he definately has a Major League fastball. Also, in our conference, every team had at least two guys who run it up there in the 90's.
So don't think just because a school is a lower division the talent pool is different, there are quality ball players everywhere.