The compressed air is probably the biggest change. So awesome. I wonder how far they can kick back. It could be use to knock back a medic on uber, return rockets to where they come from and probably get rid of those stickies easier and faster than with any other classes. I don't think I would use the backburner since it doesn't have this ability. The only one I would really use in fact is the flare gun.
A couple of retarted achievements though. Some looks very fun. Valve really needs to lower the cap for the milestone so that people can get the weapons without doing the retarded ones.