I tried playing TF2 with a bunch of mates yesterday and it's really hard to get back into after L4D. I used to play this 30+ hours/2 weeks, but I couldn't play for more than an hour without getting bored silly

Also the best disguise I find is demoman. For some reason I used to be able to do games for well over an hour solely using demoman disguise and nobody would ever suspect me, except the other demomen (whose name I probably had). GENERALLY the 'best' disguise is either the class that makes up the majority of the enemy team, or one that 'fits' in the location of where you are. For example, sniper/engineer at the 'back' of the base, demoman/pyro in corridor, pyro/soldier if there's a medic nearby you want to laugh at etc etc.
Yeah, scouts and heavies are terrible choices due to move speed + lack of minigun, but don't forget medic either. If you can't heal people as medic (and they WILL be staring right at you) they will just shoot you.