Battlefield 2 has introduced an almost RPG element to FPS.
Battlefield 2 is primarily an online multi-player FPS. Similar to CS and DOD, you are in first-person view with an easily understandable HUD and a view of your currently equipped weapon. There are a number of differences between BF2 and CS.
(1) Rank: As you play BF2, you can achieve awards and rank progress. You begin as a Private, and as you play your rank progress will go up until you reach Private First Class. You can view your rank progress by clicking on the BFHQ button at the top left corner of the main menu. Also, as you play and try to get your rank up, you will be given awards depending on what class you play the most. If you play as Support for a period of time, get a number of kills or a number of Support points, you will eventually be given the Basic Support award. Each class can be given awards.
I really like the Rank system, because it's almost like playing an RPG. The time you put into the game is rewarded with a number of rewards. It gives you rank, badges, ribbons, etc. I can't seem to quit playing the game because I want to achieve my next rank.
Also, once you reach Lance Corporal, you can unlock your first locked weapon. Some of the locked weapons are unbelievably cool. I just unlocked the DAO-12, a semi-automatic shotgun which holds 12 rounds at a time. It's a truly devistating weapon to my opponents.
(2) Vehicles: The other element of BF2 which puts it in its own class away from CS or DOD is the vehicles. You can pilot anything from tanks to helicopters, or jet aircraft to boats, and stationary anti-aircraft missle launchers to stationary machine guns.
The vehicles can sometimes become frustrating. Take it from a seasoned CS player: when I'm used to being on foot and fighting toe-to-toe with other players, now I turn a corner and I'm looking down the barrel of an enemy tank. What do you do? Chances are you won't have a chance to turn and run, because a tank missle can travel a lot faster than you can. Instead, charge the tank and hope they don't see you.
My favorite thing to do in this scenario is this: as Special Forces class, you are given Remote Detonated C4 so you can demolish enemy artillery. When an enemy tank is looking the other way, run up behind them, drop a couple C4 packs on the tank, switch to the detonator and high-tail it out of there. Detonate the C4! It's a lot of fun getting kills that way, and you'll also get Defend Flag points if the tank was trying to capture your flag.
There are so many elements to this game that I believe puts it into a class of its own.
This game is definitely worth the $39.99 (or $49.99 for the Deluxe Edition). Go out and buy it now. It's got everything a good MMOFPS should have, and some.