Jets, Tanks, large scale battles, relatively balanced classes, teamwork based gameplay, completely destructible environments.
Vs.
Lots of customization, lone wolf style gametypes/play, COD Elite, and...More gun attachments?, better killstreak organization.
Even some of the previews I've been reading are saying MW3's multiplayer is a step backwards from where Blops was. (I do not like COD but I keep my opinion out of that part of it)
Sorry, but Battlefield looks to be shaping up to be way more of a blast than MW3. This is a game that's been blowing people away in previews in demos. I continue to hear how impressive the games multiplayer is, and how detailed and fun the maps are. The class system balance has been tweaked and so far, the changes look to be smart ones.
Meanwhile, I don't hear many amazing things about MW3. The maps I have seen don't impress me, and some of the improvements Treyarch made in Blops are being completely abandoned by Infinity Ward. The game looks alright, but I haven't heard nearly as much praise for MW3 as I have for Battlefield 3.
Personally, as I said, Call of Duty is no longer a game franchise I buy. I was very disappointed by the past two titles that I can;t bring myself to try this one. Battlefield 3 looks like it has really been crafted for an incredible and unique experience online and I feel like it will ultimately be the better game.
Still, nothing wrong with buying both if you want to (those who are saying why one or the other)
I'm not going into the campaign stuff. MW3's seems to be trying to impress with explosions instead of a story (that's not a good thing, just so we're clear) and we haven't seen enough of Battlefield 3's to get a good sense of it yet. Not to mention that 90% of the people playing this will be spending the majority of their time online, some won't even touch the campaigns in either one.