I'm not talking about graphics. game play and style is exactly the same. I don't give a shit about the graphics changes. BF BC2 is just an expansion for BF2.
You need to play BF2 again. The only things that makes BF2 similar are:
Conquest Mode
Ground Vehicles
Large Open Maps
That's it.
BC2 has destructible environments (this is a huge gameplay change) via the Frostbite engine. There's no commanders in BC2. There's a 32 player limitation in BC2 as opposed to BF2's 64. I could go on, but I made my point.
However, since we're playing the comparing game... what has MW2 changed from COD4? Not a whole lot. Maps are a little more vertical, there's the addition of new killstreaks, and some of the perks have been renamed, moved, or removed. Oh yea, and multiplied all the point values by 10. Woopdeedoo.
COD4 was and still is a great game. It's just that a lot of people are sour to the fact of all the BS decisions Activision has made and all the nickle and diming that's going on. MW2 has done little to change or improve the Modern Warfare series... it's just the same damn game. People generally get pissed off when they pay $60 for the same game they bought two years earlier (which happened when people bought BF2142...essentially a reskin of BF2).
Now, mind you, MW2 is still damn fun. I had a blast playing it a few months ago on the 360, but I couldn't care less about the game any more. Why? Hackers. Maybe it's not a big problem anymore, but I've given up now, and am enjoying BC2 on the PC.