I'd played and loved Modern Warfare 2. I still believe it to be the best multilayer first person shooter I've ever played. That said, when MW2 released their so-called stimulus package at a rate of $15, I felt like that was kind of an insult to their playerbase, which helped them to record-breaking profits in the midst of a recession (particularly when 3 of the maps weren't even new). When I saw that BC2 offered new maps for free (and had already released 2), and claims some were making that it was a better game than MW2, I had to give it a try. What follows is my impressions in contrast to MW2.
When I first started playing the game, I felt a bit underwhelmed. The single player in MW2 is short, but intense. The single player in BC2 might be worth playing to get a feel for the characters, but its a pretty forgettable experience. But the singleplayer isn't why you bought the game anyway.
The sound is great, the graphics look not quite as nice as MW2 in particular with regard to weapon detail, but I think some of this is the scaling. My impression is that BC2 drew humans with a more realistic relative size, this coupled with that the maps are much larger makes everything seem a bit smaller, and perhaps not as graphically wowing. But if you look at the water, buildings, and people (up close), the graphics actually are pretty good. You just might not notice it at first.
There's also some fun stuff about BC2's personality. In singleplayer, whilst riding some 4-wheelers, they comment how much more awesome 4-wheelers are than snowmobiles. While heading into a mission, one says that they want to go in first, because otherwise they'll send some pansy special ops group with rifles and heartbeat sensors. All, of course, references to Modern Warfare 2's singleplayer campaign.
Gameplay: I'd have to say neither are 'better' than the other. BC2 is a different kind of game than MW2. MW2 is a run & gun fragging game, with BC2 there's more strategy. MW2 is on smaller maps, where people routinely camp in predictable places. BC2 has huge maps where camping is largely meaningless. MW2 lets you build a custom warrior with a wide array of addons and perks. BC2 has some customization, but not as large as MW2, and only within your class (rifleman, recon, medic, and engineer). There are no perks like MW2 has after a certain number of kills. BC2 has vehicles prominently featured in combat. MW2 does not. BC2 has destructible environments. MW2 does not. (As a MW2 player, you'll be really lost at first 'How the hell am I supposed to take cover? Everything is exploding!').
I also really love what they've done with the sniper, calling it recon. This makes the role more useful than some griefer only contributing a few kills, because they can call out enemy positions they observe through their scope and place motion sensors.
There are some minor gripes I have with BC2. Such as the repeating sound on the unlock page once you unlock everything for a class, the small and difficult to read menus when not displayed on a widescreen TV, the awful guns you start with in assault class, the unskippable cinematics when you start the game and at the end of each round of multiplayer.
They do seem to have taken a cue from MW2 when it comes to unlocks. You'll be able to win new guns and gadgets as you play along in multiplayer. The pace seemed slower to me than MW2 because there weren't a lot of little things that you unlocked like MW2. But many people find 'the hump' less than MW2. I'd say the guns are more balanced overall (with the exception of the M80 for the medic class). One thing I don't get is the pins you win for doing things like getting a revenge kill, being thrifty with your LMG ammo, etc. They award them to you, but you can't really do anything with them. It was nice in MW2 to be able to brag about them by putting them up with your nametag.
What others have said about BC2 being more strategy and team oriented is true. You can't win the game by yourself. While they do have a deathmatch, I play (and most others too I think) rush or conquest, mostly rush in my case. It doesn't matter if you go around slaughtering the other team (although I think you'll find that more difficult to pull off in this game). If your buddies don't have your back, your team will still lose (and you will probably die).
I didn't feel like BC2 was a better game, especially with respect to single player. And while instinctively I'd say buy MW2 before buying BC2, I also have to recall that since getting BC2 I've barely touched MW2. I certainly don't miss the camping, lag & DCs. If you have a preference for fragging games, get MW2. If you enjoy multiplayer shooters that involve a little more strategy, get BC2. If you've got enough money, get both. They each have something different to offer, and having both provides for a good change of pace.