Defence is the best form of attack. Is that right? No: attack is the best form of attack, and the engineer's turrets are the best form of defence. So where does that leave the Demoman? Well, he's the midfielder of death. Neither truly an attacking class or a defensive class, he's just lobbing damage into the middle of the map and hoping for the best. Most of the time, of course, he's coming up trumps: how many times has your demise been a grenade in the face?
No other class quite boasts the versatility or the punch of the Demoman. The grenade launcher has two possibilities: pumping out explosive charges, or placing stickybombs,
either as fire-and-fergedaboudit mines, or as traps to be detonated later on. As Demoman you're most likely to be an unsung hero – who else is going to be able to turn a corridor into a death-trap at the squeeze of a mouse-button? Who else bounces a grenade round the corner to take out a turret without risking a follicle on your cell-shaded scalp?
Neat calculated trajectories aside, I suppose playing the Demoman is largely what we'd call 'spamming'. As soon as I started pumping out the Demoman's grenades I was flashing back to Quake III team deathmatch – sticking grenades all over the place, just to keep the enemy team at arm's length. You spamming bastard. Not exactly a precision instrument, you're just putting out so many tumbling bombs that your foe generally isn't going to escape – and at close range you're probably for it too.
But there are other, rather more interesting side-effects of being a Demoman. The first is the to ability sticky-jump. Now, I've not played enough to really have found a tactical necessity for this, but being able to get a good sticky jump in has always been one of the joys of a fortress game. Getting more than one person airborne on the blast – well, that's the art of it. A rooftop vantage point can make those precisely lobbed bombs all the more dangerous.
The second, well, it's a little more subjective. It's the sweet fluke of the bouncing bomb. There's a kind of joy in the randomness of getting a kill you weren't expecting. As almost any other class, you're going to fail if you don't put your ordnance where it was intended to go, but with the grenades, well, you can end up landing them in path of speeding scout, or putting one through a window just as a sniper steps into the frame, or simply dropping them down from a vantage point as engineers are scurrying to set up their defences. Bounce, bounce, death. There's an epitaph worthy of any gamer's tombstone.
Finally: the ultimate fighting retreat. Holding off an advancing medic/heavy combo, because you're the one with bombs. Because that's the way the Demoman rolls.
The Demoman isn't the class I'd want to end up with if I were somehow contractually tied in, but hell, he'd be a good second choice.