Unique, fun and satisfying with an overall appealing multiplayer community. Team Fortress 2 is worth the purchase.
Some players can be put off by the idea of relying on other humans for assistance and in essence, this is what TF2 is all about. I've gone through server after server and I can tell you, in general, the majority of players are there to have fun and play the game as much as you are. The only likely negative outcomes that I can think of are the odd player who disagrees with the tactics the rest of the team are using or one who will not strategically choose a helpful class at the current stage of the game. So no big issues. Oh, a headset is handy as in fast paced situations, it's not so good to be stuck typing.
There are some very nice gameplay mechanics that come with each of the game's player classes, such as The Medic's ubercharge. The ubercharge is a device which builds up on a medic's "healing" weapon and when it is fully charged, it can be used on a friendly player to make them invulnerable for a short period of time. You may think that this is an unfair God-like advantage. It is not. This is where another of the fine classes comes into play.
The Pyro. Setting the enemy on fire, what fun. But this class is used for much more. For one thing, the pyro's flamethrower includes a secondary use, the compression blast, capable of blowing back enemies (such as those currently using the ubercharge - to stop them from destroying your team's "buildings" - more of that later) and also blowing back enemy demoman's stickybombs.
The Demoman. If you like to make things go Ka-Boom, this is for you. The Demoman can either fire small, bouncy explosives or plant stickybombs (strangely enough, bombs that stick to things) to turn their enemies into small pieces. These are also very useful for destroying building built by an engineer.
The Engineer. A great class here, which technically falls under "defense" but can also be used for attacking and support. Not only can the engineer build dispensers to, well, dispense ammo and also health to nearby allies but also can build sentry guns to protect control points and strategic locatons. The engineer can also contruct a teleporter system to get your team from the spawn point to the frontlines in 3 seconds flat (when fully upgraded). Dispensers are great for keeping heavys going. Satisfying to play.
The Heavy. Commonly known as "fatty" is the large character that you often see eating a sandwich, sorry, sandvich. But don't be fooled, when combined with a medic feeding health to him (he had extra hp) and a dispenser feeding continuous ammo to his large minigun (not so mini), he's a deadly force to be reckoned with.
I don't want to drone on about every class but let me tell you that the Soldier, Spy, Sniper and Scout are also fun to play when used correctly - which is easy enough to work out because the game gives you hints and tips at the selection screen and on loading screens.
With a wide array of custom maps (my fav being "lazytown_lazyday/nite") and distinct personalities for each class, Team Fortress 2 quickly became the funnest game I had played in a LONG time. With support updates for each class and patches being churned out even today to improve it further, TF2 easily gets top marks from me.