I've played very little of either, but it seems to me Battlefield has everything COD has but with the extra awesomeness of the use of all sorts of vehicles, particularly in multiplayer.
So what is it that makes COD so popular?
...then whoa, differences...
The two cant really be comparred. BF focus on veichle combat and big map warfares useally with objectives (capture zones or destroy something), and a lot more teamplay orianted. COD on the other has smaller map and focus on quicker gameplay, and alltough some game modes include objectives most player focus on fragging.
Combat and gameplay is very different in the two, even if they may seem the same.
"I told the others, they didn't believe me. You're all doomed. You're all doomed."