"What makes me a good demoman? Well if I was a bad demoman I wouldn't be sittin' here discussin' it with ya now would I?" Truer words have never been spoken, and if you expect to succeed with this class it means playing defensive and being a very very good shot with the pipe bomb launcher.
You are not a straight up fighter
The demoman does not have any direct fire weapons like the shotgun, everything is indirect, and unless you are very good at aiming the pipe launcher and hitting people head on or planting stickies at your feet as you fall back, you might want to run away at times. The demo is inherently better at defense than offense. However, you make one hell of a hallway fighter, very hard to avoid the pipes if you cant strafe left or right.
Don't stack all your pipes on one spot
Spread your pipes out in a trap, it gives you more leeway when detonating them and can make things more inaccessible, for example in Granary like maps you want to spread the pipes out across the whole cap and not just right in the middle.
Prancin' aboot with yer heads full of eyeballs
Sometimes you want to hide your traps by placing them around the edges of doors and other times you want them to be visible. Visible pipes can be a great way of discouraging players from entering an area. The player has no clue if they are being watched or not and you can get a big pile of people not wanting to be first to test the pipes in maps like dustbowl.
Use more air pressure
Charge the sticky launcher to fire them longer and with a flatter trajectory, most people should have figured this out by now :)
A cluster of pipes can be used to knock the medic away from his target or the target away from the medic. This is most effective in maps like dustbowl where an uber may be used to rush a cap point. Kill entire defensive setups with stickies
Due to their blast radius, stickies can be used to take out sentries in one shot, especially hard to kill ones like the basement of 2fort or certain spots in dustbowl. Regular pipes work as well and a volley can end up killing an engineer, his dispense and gun all at once.
Be like Ron Popeil - Set it and forget it
Pipes, especially those on a cap point or flag, don't need to be attended at all times, put the pipes down and go forward and spam. There is a risk though, if you are killed what ever you had pipes on is now defenseless.
What do you mean eeuuughhh! I DONT LIKE SPAM!!
Shoot your regular pipes anywhere and everywhere, thinking every time you are about to fire them that someone is going to come around the corner. You can never have too much spam, ever, its simply not possible.
While it may not be the spam machine it was in TF/TFC because it lacks MIRVs (god I miss those so much), you can still effectively spam corridors, and actually set up better traps by being able to hide your pipes on any surface. Just try not to lose your one remaining eye by fumbling explosives.
Spy Medic Sniper Pyro Engineer Scout