I'll keep it simple- Team Fortress 2 is a blast.
Team Fortress 2's main gameplay is very similar to what was in the original Team Fortress. There are 2 rival teams fighting each other in a variety of modes- such as CTF. When you spawn- you get to choose one out of 9 comical classes: The scout, the pyro, the demoman, the heavy, the medic, the sniper, the spy, the engineer, and the soldier. Each class is geared to have specific strengths and weaknesses. For example- the scout is by far the fastest class and can capture control points twice as fast as other classes- but he's also one of the most fragile classes. The classes balance each other out well- so you'll need a well balanced team if you want to fare well online.
While the online community is unsurprisingly very active- Team Fortress 2 doesn't have all that many maps available right now. But we already know that Valve is hard at work solving this problem- making brand new maps. So just be patient. New players unfortunately have to brave the online environment right from the get-go, since the game doesn't offer offline bot matches. But if Counter-strike source was any sign- we may be seeing bots for Team Fortress 2 relatively soon.
Graphically, Team Fortress 2 is seemingly primitive and cartoony. But this is intentional- and it fits the themes that Valve was going for perfectly. Sure- the weapon models are a bit plain- but it works. The environments manage to be sharp and cartoony at the same time fortunately- so the source engine does have some heavy duty work to do in TF2. Sound wise- Team Fortress 2 has some well done sound-design, especially the voice acting of all the characters which is fittingly cartoony and silly. These guys could easily fit into a Pixar production (Yeah- I don't think anyone has not heard that comparison...).
All matters considered- Team Fortress 2 is like an old childhood friend you get to reunite with. But your friend has matured... And you like him/her even better now! If you played the original Team Fortress- you should be able to get right into TF2. Those who have played other online shooters before should also be able to easily enter and enjoy Team Fortress 2.
Pros: Old-fashioned multiplayer gameplay that still works excellently. Cartoony graphics are a joy to look at. Great voice acting.
Cons: Not that many maps available (right now...). No bots available (yet...).
Recommendable for: Team Fortress 2 is one of the best online shooters available right now. Anyone interested in one should definitely check out TF2.