Joe,
by far the biggest difference is in recruiting:
1) prestige is way more important
2) lots more money spent on each player. When i was in d2 and d3 i'd read threads where people said they did 40 HVs and 20 CVs, and think that was insane. Now I do that many all the time, think nothing of it.
3) lots of opportunities at the lower end of prestige to wait on recruits dropped by bigger schools.
4) as reinsel said, you can rarely afford to miss the beginning evening of recruiting (though oddly, you might be more easily able to do so at the lower levels).
there are many more things, these just pop to mind right away.