Scouts (good ones) are amazing at defense. I love playing defense as a scout. You can get to a base and stop a capture before anyone else, and your speed allows you to really sidetrack and annoy people. It's amazing what people will go through to go after a scout rather than the primary objective. I've actually held off half of a team before at a choke point. Not because I was amazing, but because my hit and run attacks just scared the crap out of them.
NO. Rushing in and dying is a horrible waste of your abilities and time. Soldiers are the second slowest class, so it takes forever to get back into the fight.
Soldiers are amazing at taking out buildings, getting to hard-to-reach places, and laying down concentrated damage. Find a choke point and clear it out.
Also be warned that a couple of ubered pyros can totally change the tide of war. Do not underestimate them, they can own you.
Always take out the dispensers first. Engineers have a fit and are nearly useless without metal to rebuild. If you can take out a turret and dispenser at the same time, all the better.
On some rounds, heavies can be amazing at defense. Stick them next to a dispenser and just let them lay down an infinite stream of lead. Doing this makes them one of the best spy-killers also. If you cover the area properly, spies can't get through without blowing their cloak and taking damage.
Also, when staring down the barrel of a sniper, CROUCH. They'll blow their shot into your chest or above your head, giving you a chance to get some cover.
Always, always, ALWAYS build a tele entrance at the spawn point, then run back for a refill of metal. After that, always, ALWAYS build a dispenser. They'll allow you to get that sentry up much quicker, and benefits the entire team.
Don't be afraid to be a little offensive with your placements. A sentry placed in a choke point can put down a lot of heat. Always build near ammo spawns if you can, so you can easily grab extra metal. Once your dispenser has topped off, you can repair your turret for a nearly-indefinite amount of time.
Do not forget demos. On a couple places in dustbowl, the only way to really make it forward is to have a couple demomen ubered to rush through and take out the entrenched team.
And I also say that medics SHOULD run into the fray, and often. Medics are the second fastest class, they can dodge shots really effectively. Combine that with their regenerative ability, and they're like a fast-moving tank sometimes.
Only bother sniping lvl1 sentries and dispensers, which you can take out with 1-2 shots.
Pay attention to the charge meter. Even a headshot is not enough to kill a heavy at a low charge.
Also, as far as spies, other common choices are demomen and pyros.
I refuse to acknowledge the existance of this broken class.
No but really, spies are insane and can really screw up a team.