Battlefield 2 is an improvement over 2002's game of the year Battlefield 1942. Sure it did not win any significant award as it is clearly intended to the pure fans of Battlefield series. You won't feel that good surprise that the good old BF1942 brought when they first came out.
BF2 however, bring tons of wishlist to reality, especially in playing department. So it is not just another plain short semi-RPG MP game, where a bunch of players are crammed together in a working map and players being empowered with whatever GUI/HUD/interfaces the engine can provide like BF1942. It is basically, transforming it into a fully FPS game with all Battlefield gameplay goodness.
Your player character interaction is greatly enhanced over BF1942:
You can now sprint in distance like pretty much most dedicated FPS game like Call of Duty, Medal of Honor.
Your gun has as much customization as you want just like dedicated FPS game, it is no longer "just a boom stick", you can change its rate of fire, you can actually see the animation, multiple choice of guns for a player class (ex. Medic can choose between L96A1 or M16A2 using achievement unlock feature).
Interface improvement: wanna say something? Forget the F1-F8 key, introducing the commo rose.
More empowerment while in vehicle: you can pretty much fire from your vehicle, knife, fire you guns, throw grenade, or simple things such as switching your gun... changing vehicle position is now binded to F1-F8 keys.
Team work: form a squad, and get some benefits from making a team.
Think while you play: now you can execute any brilliant strategy by being commander. It is now more like the military, less like simplistic FPS game. Deal with bratty players, egoistic squad leader, or blind commanders. Commanders can give order, which everyone can see in their map, call reinforcements, attack order, dig-in, drop supplies, vehicles, even calling in arty strikes!
Something for you: hoard points, advance through ranks, and get priority when applying the commander position! Or access to extra contents such as alternative weapons. You can help your teammates for points, help the team to achieve something like capturing flags to earn more... so you don't always have to be the ultimate killing machine to be on the top.
Class customization: instead everybody being same exact soldier, now they are really different, support class carry MGs, extra ammo, and more body armor, therefore they are harder to die and sprint short distances. While spec-ops class, basically modern ninja, carry less and wear little armor, they move fast, sprint further... well all that stuff logically.
However, I do feel that BF2 somewhat has several weaknesses compared to BF1942. Namely...
Tanks!!! They have this kind of "rubbery" "plasticky" feel physics built into them. Unlike BF1942's tank, which feels more like metal, BF2's tank somehow defy physics in a way that they are more like Need For Speed cars when in collision (unless we are talking about BF1942 tank's wobbly suspension, which makes Tiger tank like a giant see-saw) Sure they look and sound cooler with all updated graphics and sound engine, even FH2 tank did not escape this disappointing drawback. IIRC, BattleGroup's tank physics is more like it, but it is very buggy.
Airplanes. Yes, almost every vehicles in this game feels worse than when they are in BF1942. Probably it is a trade-off. You got fabulous infantry interface and interaction, while vehicles is... well, alright. The planes has this poor physics that makes them fly perfectly straight like it is some outer space battle between Tie-fighter vs X-wing... literary, cause the gravity just don't work that good. Every hard maneuver won't get any return feedback (the one you clearly feel in BF1942 and of course, Il-2 series), you'll just fly the way you press your keyboard.
Well, did I mentioned poor physics? BF2 simply blows it compared to BF1942... because ships isn't modeled anymore. Perhaps I am right, they are meant to be enhanced FPS game. You know, unlike CoD or MoH where you can't drive tanks proper or fly jets. This has to be smoother and less clunky than ArmA or Op:Flashpoint... meaning, it should be more action oriented.