Trailer 2

Info

First revealed at GC '06, this trailer showcased the various abilities of Team Fortress 2's player classes.

The Scout is shown here wielding a Nailgun, which was replaced by the Scattergun in the finished game.

The Demoman is shown wielding and throwing a MIRV, a would be stay-over from Team Fortress Classic, which was removed from the game following beta and replaced with Sticky bombs.
The Demoman's Grenade Launcher could hold up to 6 grenades.
The Demoman's ability to remotely detonate his Grenades hadn't yet been moved to his Sticky bombs.
The Grenades don't flash like they do in the final product. They also seem to be longer and thinner, with a slightly different design.

The Heavy's Minigun firing animation includes a more cartoonish muzzle flash coming out of the barrel. The final version's lighting appears more realistic.
The Heavy's Minigun fires bullets in an extremely wide spread.
The Heavy's movement speed is much slower in the finished game.
The bullets seen spewing out of the Minigun are larger and have a different design. There also seems to be more bullets being expelled than in the final product.

The Level 1 Sentry Gun has a different firing sound, which originally seemed to be the MP5 sound from Half Life, builds at quite a slow speed (But this may be because of the fact the Engineer isn't hitting his Wrench off it), and seems to have a higher rate of fire. It also has a muzzle flash, unlike the simple recoil the gun has now.

The Sniper's animation when backstabbed was later implemented as a unique death animation.
The charge symbol seen when the Sniper Rifle is zoomed in has a different design.

Spy

The Spy's Knife has a much longer and more intricate switching animation than in the final game. Also, in the trailer, the Spy could hold the knife before deploying it.

The Pyro and the Medic are both seen in the trailer, but neither are mentioned.
The design of the characters is very close to the finished game, but there are still some slight visual differences.
Footstep sounds were recycled from Half-Life 2.
The classes all have their beta designs which are all still seen in the Heads-up Display while playing.
The emblems on the class' arms are solid circles instead of class emblems.
Weapons were less jerky when being wielded by classes. For example, the Nailgun held by the running Scout was almost stationary when it was viewed
The spawns have a separate exit and entrance like in Team Fortress Classic.
The 2Fort bridge is uncovered.
The explosion graphics are different in the finished game.
The picture showing all the classes at the end is an old version featuring the older designs and weapons.
All characters have the same voice and stock dying noises including the infamous Wilhelm scream.
Characters have a few clarity issues. For example, the Heavy's ear and Demoman's eye patch are a bit blurred.
The finished game has far more blood when players are gibbed.
The classes' health were set very low (especially Soldier and Pyro) so that the other classes could kill them easily.