I've been playing a lot as a demoman the last few days and have been racking up the kills. Anyone that is just playing defense as a demoman isn't understanding how truly powerful he can be.
First, besides spies (which we all know require a bit of luck), demomen are the best way to take out sentries. All you need to do is figure out where a sentry is and toss 8 sticky nades right next to it. Since you can remain hidden while doing this, you won't take any damage. Once you have at least 4 nades out there, blow them up.
In some cases, I like setting up 2-3 sticky nades around me before attempting this. That way if someone sees me getting ready to take out the sentry and runs at me, I can back up and take them out along with the sentry.
Second, on defense a demoman pairs perfectly with engineers. The only thing engineers have to worry about are spies and ubers. So, put a couple of sticky nades around their sentries and wait until you hear the "spy's messing with my sentry" call. Blow the stickies and you'll take out the spy, so the engineer can just worry about fixing his sentry.
As for ubers, you'll have plenty of time to react to these. The main thing is to avoid being killed and to get your sticky nades onto the CP (or nearby chokepoint) as quickly as possible. Just don't get killed! You're not going to do any damage to the uber'd medic and heavy/pyro anyway, so you might as well hide until it wears off. Unless the other team already has started capturing your CP, it's impossible for an uber duo to capture the control point. Just wait for them to get on it (preferably waiting until the medic is on your stickies) and set them off. You'll probably kill both, but as long as you take out the medic (or knock him away), your team should be able to finish them off.
Third, play defense and offense at the same time. Set up some sticky nades on your control point as you run by. Once you get to the enemy, start tossing normal nades. This only really works if you have teammates around that you can sit behind. Your main goal here is to try damaging the enemy to make it easier for your team to kill them. If you hear your CP being captured, set off those stickies back there.
Fourth, if you're close to dying, set off your stickies no matter what. You never know if there's a spy nearby them and you might as well not waste them.
Fifth, when placing your stickies, spread them out a bit. Some control points are pretty big and it's easy for the enemy to avoid your nades. Just put them in the areas the enemy is most likely to stand on (the very center and out toward hidden corners). You can't defend the whole area, so just do your best to damage anyone trying to capture.
Finally, certain control points are impossible to protect unless a good chunk of people are playing as demomen. There's one on Hydro (the one in the basement area?). The only way I've seen people defend this is by having 4-5 demomen lay stickies all over the place and set them off when people come in to capture. You can give your engineers time to get a few sentries up and then you have a fighting chance of defending. Just be careful that all the demomen don't set off their stickies at the same time... stagger them a bit so you're never caught off guard.