I've played most games from both franchises, so their unique qualities are well-known to me. At the most basic level, previous BF games have appealed to only one kind of player: gamers looking for a first-person, competitive war game experience. I use war game instead military or tactical shooter because the BF franchise is much more than that. Since BF1942, combined arms operation has been an integral part of the BF experience. An infantry only assault could be easily shattered by a joint force of armor, artillery, and aircraft in addition to infantry. Yes, BF1942 even had playable self-propelled howitzers. The sheer scope of the battles has not been matched by any other first-person war game except Operation Flashpoint and its successors.
Unfortunately, BF1942 and BF2 had such powerful focus on their dominant Conquest mode that every other aspect of the games atrophied. The most glaring omission from the franchise's main entries is the single player. There is no single player campaign in BF1942 to BF2142. Not only that, the support for single player practice on multiplayer maps against AI also got progressively worse over time. When the series reached BF2142, it basically turned away all players interested in anything other than competitive multiplayer play.
On the other hand, an engrossing single-player campaign has been a trademark of every Call of Duty game since the first. Crossing the Volga in a helpless ferry while Stuka bombers are blowing the **** out of Red Army soldiers all around you was just as intense when played (maybe more so) as when watched during the movie Enemy at the Gates. This was only the first game. Subsequent games had even more over-the-top yet somehow believable stories filled with heroism, conspiracy, and the good old Red, White and Blue. When I compared the lackluster EMP detonation in (I still don't know the city) from Bad Company 2 to one of the defining scenes of MW2 taking place in space, it made me feel sad to even be playing BC2 single player.
To be honest, I don't think any semi-modern, semi-realistic war game could have a more extraordinary story than Modern Warfare; it's as over-the-top while remaining sane as it gets. The only thing that may be able to surpass it is real war. Ultimately, war gamers are attracted more to the grit, the camaraderie, the terror, and the sheer determination of real battles than any level of action a game could attain. BF3 appears to have done that, using the most defining war of this generation, a war that even now spills the blood of young Americans over the sands of the Middle East. If DICE executed this premise correctly, it would have created a story that no Modern Warfare game could match. Ironically, this was the kind of story in the original Call of Duty, not of heroes who performed seemingly impossible deeds, but simply of men who relied on each other to carry through the darkness so they could see the dawn of another day.
Such a story would put BF3 on par with MW3 in terms of single player. Then we are back at the multiplayer portion of these games, an area where BF3 should have a decisive advantage. The problem with small-scale infantry shooting games is just that. You are either good at infantry level shooting or you are meat for others' kill count. You could always aim to get better at it, but to both casual gamers and war gamers alike, diversity is better. I myself do not care to be good at shooting; I shoot ok but use medical and repair skills to assist my team. Some people want to drive tanks. They might shoot 10 out of 100 on the range but if they could dance around incoming RPGs and pick off a sniper on a 50 meter tower with their main gun, they are still invaluable to the team. Others who have joysticks may want to fly jets around the battlefield, both supporting ground units and engage in air-to-air combat. Still some may just want to drive a truck or pilot a transport helo to ferry troops all over the map. The beauty of combined arms warfare is it needs many different specialties and permits many different kind of players to contribute. This is much more attractive to casual and war gamers than shooters because no matter how well made the game gets (such as CS or MW), if you sucked at shooting, you suck at the game and no one would want to play with you.
There isa way for BF3 to surpass MW3, and it looks like the game has set off on that path. Whether it has taken a successful first step remains to be seen. In addition, Infinity Ward will not be standing by while DICE surges forward. However, that's in the future. For now, Peter Moore appears to be fully capable of leading EA and DICE to take advantage of the opportunity to expand BF3 while MW3 has remained somewhat stagnant.