Basically, the heavier classes carry bigger backpacks. Also, the running animations aren't very good. The models sometimes seem to float more than run. There are 9 official FLF maps released so far, and plenty of user-made maps are in the works or already released. However, overall the maps are very enjoyable and extremely well-made.
Some maps are defensive, some offensive, and some are just plain pretty.
Bier Attackers try to take over several points in a grimy city and can capture the command points in any order. The Living Room point sees some outrageously intense action. Kind of nice when you are defending that point to sit there and groove for a while, at least until the lead starts flying. Bridge Attackers try to take over several points in a town. Points can be capped in any order.