The Demoman
The Demolitions Man (Demoman for short) is a crazy freak who runs around with a large number of explosives. Demomen run at average speeds, and can have 90 health and 120 yellow armor. His primary weapon is a grenade launcher which can fire grenades or pipe bombs. The pipe bombs act like normal grenades, except they will not explode until the Demoman detonates them or an EMP grenade explodes near them. The Demoman also carries MIRV grenades. These grenades explode once to scatter 5 normal grenades, and then the 5 grenades explode simultaneously. MIRV's are great for clearing a room or keeping people out of a hallway.
Even with such potent firepower, the big weapon for Demomen is the Detpack. Detpacks are the most destructive weapon in the whole game. They're primed and set on the ground, and primed to explode by a variable timer. Anyone caught in the blast radius of a Detpack will instantly be killed. Detpacks are also used to set off triggers in maps, such as opening a wall or blowing up some rocks.
What's New?
The physics of the grenades are now totally different from TF1. First, the grenades are much more elasitc, and bounce around a lot. For example, if you shoot straight down, the grenade will actually bounce back above the level of your head. The number of times that the grenades bounces is also more than in TF1. What's odd is that the grenades actually fly in the air slower but farther in TFC as well. I don't know how these things work... maybe the grenade cases are made of rubber? Who knows?
Pipe bombs are slightly different also. They bounce just like the normal grenades, but now there is a very noticeable lag when you try to detonate them. It's not a connectivity issue, but rather a built-in delay. I asked Robin Walker of TFS/Valve about this last night when we were playing, and he said there was nothing they could do about it.
Overall, the grenades (hand grenades, grenades from the launcher, and MIRV grenades) hurt more than in TF1. The blast radius is larger, and more damage is dealt per explosion. MIRV Grenades do a massive amount of damage over a rather large area. In addition, you can still grenade/pipe jump as well.
As an aside, the new detpack model looks neat =]. It looks look a liquid mixture explosive a la Die Hard 3.