Another awesome shooter from Valve.
For the most part, TF2 sticks very much to the mechanics of Team Fortress Classic. All the same classes are there, and while their looks and mechanics have certainly been changed a bit, the roles that they play are mostly the same, graphically enhanced about a decade thank you Source Engine.
Each class is a very different playstyle, and with the possible exception of the Sniper, none of them are super intensive on the kind of aiming that DoD or CS:S require. Instead, they all require a different set of skills, usually the ability to know how your class can get the better of other classes, leading the enemy with your projectiles etc. Basically,there's not a lot of "click and instantly hit your enemy" action. Combined with a near-disregard for certain body parts=more damage, this game shifts the gameplay deep into the realm of teamwork=win, solo effort=lose.
The core game consists of a pretty big variety of maps and gametypes, ranging from capture the flag, capture point warfare, capture point defense/offense, and moving the cart from point A to B. Along side these you can easily find dozens upon dozen of fan made content, just like any other shooter.
The gameplay is competitive, but not usually so much or so hostile as a lot of other shooters, just because the more cartoon look and feel kind of seem to diffuse things a bit. Things can get heated, but there's a lot of different ways you can play the game, so there's usually good solutions to any problem, provided your team is behaving intelligently enough to cooperate and get things done together.
A couple of years after its initial release, Valve began releasing new weapon varients for the various classes that added a few new interesting elements to the game. Some of these are noteworthy, and others fairly useless, but it added a much needed note of variety to a system, one in which you no longer knew exactly what to expect from someone, because their loadout could very much change how they were a threat to you and your team.
Fun for everyone, a class for everyone, what more could you ask for? If you haven't given this a try (And I would suggest picking up the PC version instead of the 360 version :)), and have an affinity for shooters, I'd say you're missing out.