Soldier
The all-rounder.
Advantages
Armour and firepower.
Disadvantages
slow, Rocket launcher needs frequent reloads
Health
100
Armour
200 (spawns with 100)
Typical role
defence or offensive support.
Notable Weapons
Rocket launcher
Delivers lots of damage to medium ranges very rapidly. Splash damage can make it very effective against lighter classes such as the scout - even a near miss can kill them. The RL is also very effective against sentry guns, especially at longer ranges.
Nail Grenade
A very effective Sentry Gun killer, and also useful for area denial purposes.
Offensive capacity
Lots. The soldier's rocket launcher is a deadly short to medium range weapon, and the only real problems at longer ranges are that the enemy can see it coming, and the general difficulty of hitting moving targets. Lack of speed can make the soldier a poor flag carrier, but he can be very useful in a supporting role - engaging and distracting defenders, as well as destroying sentry guns.
Defensive capacity
Again, considerable. Positioned so that an enemy has to approach the soldier through a bottleneck, the soldier will get at least one good shot, which will do considerable damage.
How to fight them
Fast classes can dodge their rockets and try to run past while they reload, though this will rarely work against a good soldier. Slower classes must slug it out. Once you get past a soldier, most classes can outrun him. Alternatively, classes with powerful short-range weapons (e.g. nailguns or the flamethrower) should get very close - many soldiers will be reluctant to fire the rocket launcher at point blank range for fear of hurting themselves.
Tricks:
The Rocket Jump
An old favourite in many FPS games. Look down, jump, fire the rocket launcher at the ground, and soar to high altitude. The downside is that you will take some damage. The upside is the ability to take shortcuts when moving around a map, thereby bypassing defenders or reaching areas they don't expect you to be able to reach.
Play Pinball
Blast from the rocket launcher can be used to push enemies around. Particularly useful when they are standing near a big hole, or attempting to run along a narrow catwalk.