Okay, debriefing...
Only 2/4 playable classes in the demo: Scout and Soldier. I'll go through the Scout first, and highlight general aspects of the game while doing so.
The Scout, as the trailers indicate, can turn invisible. But, you start out also being able to do a quick Dash. You can further more abilities, the other one in the demo being a fake item drop that stuns enemies. You can customize your layout to have two such special abilities, so you aren't even required to have Cloak.
Vehicle mode is very important. You are safe while Cloaked, but once you get out of it and are on cooldown, you are a low-health sitting duck in robot mode. So you turn into a sports car, where you are much faster and much more mobile, with the added bonus of a low profile. You also have a decent machine gun in sports car form, and can circle strafe enemies.
So, as a Scout, you're relegated to hit-and-run attacks or sniping (you can use a sniper rifle, yay). Your place is not on the front line... which is where the Soldier comes in. The Soldier has slightly under twice the health of a Scout, and can fit guns such as the typical assault rifle, shotgun, and/or minigun. As a Soldier, you get a melee whirlwind attack, which does a shit-load of damage, but makes you an easy target for 5 seconds to anyone that manages to get out of the way. You are also very, fucking, slow... So you turn into a tank, that is also slow, but can fire a massive cannon. You can also "boost" for short durations and thus move from place to place faster than in robot form. Again, you are low-profile, so in combination with the boosting, you are a much more evasive target than in robot form when you're trying to get away.
My main qualm with the demo is that I feel like the map is too small. There are 7 more maps in the full game, so hopefully at least one of them is a large and results in less frantic and more thoughtful gameplay.
Demo vid I found
http://www.youtube.com/watch?v=dRzBTE1GacI
Edit: Oh yeah, multiplayer modes.
Team Deathmatch
Conquest (just like it sounds)
....were available in the demo.
Available in the full game are:
Some sort of CTF I think
Plant the bomb
Escalation, which is team vs horde of AI.
And the other 2 classes are support classes:
Leader. Think Optimus. He can throw up buffs and Shields, and turn into a truck.
Scientist. Combination of Starscream and Ratchet, I guess. He can support offensively or heal allies, and turn into a jet.