Team Fortress 2 has many different aspects to it than any other FPS game. One of the many features that TF2 delivers is the importance of teamwork. In public large CS:S servers, hardly any strategies or teamwork plans are laid out, just run out and shoot. If you want to win, be the last man standing. Over in TF2, team is in it's name. Everyone needs to work together, in choosing the appropriate number of classes, when and how to rush out and listening to others. You really need to do your part in engaging in other's parts for your side to succeed. The reason I - and I'd wager most people who love TF2 - love TF2 is because the emphasis is on the team over the individual.
One major difference between TF2 and CS:S/DOD:S is aiming. It's really easy for beginners to pick up this game and start playing. Anyone who's played TF2 and CS:S will tell you that this game is not so much about aiming. Someone who has first started playing CS:S will join a server and go 0-20. Now, has the majority of his game time been actually playing or waiting, watching someone else? That tends to be a little boring for a beginner, but TF2 is like the DeathMatch mod on CS:S. Depending on the server, the wait times are different.
It's the focus on the team that makes it fun. But not every class is entirely team-oriented. Snipers, Scouts and Spies - while they may make a legitimate and useful contribution to a team - are not exactly team players. They often are expected to work alone. It's just their class role. Other classes are entirely dependent on support and work with others, and rather than just running around really fast, are constantly in the thick of battle.
TF2 includes a variety of different classes (nine, if you will) that require different game styles. On CS:S, you're either T or CT. On DOD:S, they're all shooting some sort of gun. (I don't play DOD:S so I can't make many comparisons to it). here are nine classes, each of which is fundamentally different. Try all the classes, there are good odds you'll find one you like.
Often times players enjoy coming into a TF2 server with their primary goal of having good company with other players and earning achievements as their secondary objective. CS:S does not have their own set of achievements, but on TF2 it pushes you a little to play a certain class, to get the unlocks. That's what keeps you playing, like leveling up in a MMO.
I really do hope soon enough you'll purchase TF2, as it's totally worth it. I spent $9.99 during a holiday deal and glad I spent every penny. Bored of one map? Move on to a different game mode, as there are lots to enjoy.
