For me, it's easily the content that came with the Aftermath expansion. Due to lock-on warfare and the often indirect nature of gadgets, I despise Battlefield 3's combined arms balance - infantry, land vehicles, chopper and jets just don't mesh in a way that's fun to me - battle often feels impersonal, like it's not been a fight. Aftermath's only got infantry and land vehicles and scout choppers, all which mesh together in a more personal fashion, and thus are much more fun to me.
Markaz Monolith is the most promising map in Battlefield history, and quickly became one of my all-time favorites. It's unprecedented verticality and spot-on mix between wide-open and tight spaces is simply amazing, and I feel like it's just the tip of the iceberg, as how far DICE will be able to go with urban warfare. Full-blown vehicular mayhem in a true metropolitan setting - here we come!
Since BF:BC 1, I much preferred Rush mode over classic Conquest, but something about BF3 made me sour on the formula. Likely it's the blinding lighting. Being locked into attacking or defending seeing nothing but sunglare makes my blood boil with rage. It also took but one mortar to the head, whilst disarming an M-COM, by a dude on the other end of the map, that I knew Rush in BF3 is no longer for me. Shame.
With that in mind, core rules large version maps 48-64 player Conquest is how I chose to play BF3, and Aftermath's the only expansion that doesn't get me foaming with rage from the mouth (outside of the utterly inoffensive Close Quarters maps, which are baller infantry-only maps - but thusly don't offer the full Battlefield experience). I also enjoy Back 2 Karkand and Armored Kill, but every so often some aspect of the combined warfare balance gets me so angry... best not to talk about it. Something about End Game really gets to me, more so than other expansions. A map like Kisari Railroad should totally be in my wheelhouse, but BF3 just manages to make me mad more often not, playing End Game maps.
Vanilla works nicely for me. I think all vehicles as well as infantry gets the opportunity to shine.
I like stabbing people from the front as opposed to stabbing them in the back. It is way cooler.
I like welding enemy tanks/vehicles with my repair tool, and as they jump out, I jump in and steal their tank/vehicle. I hardly use them, so I almost immediately jump back out again. The enemy either tries to run away, blow up his old vehicle or jump back in as a driver as I usually jump out again in which case I repair/dismantle his ride. Either way I win.
I also like to attack & clear buildings. If a flag is inside or near a building then I'm in my element. I hate defending buildings and tend to move on to the next flag instead of just camping out.
My newest hobby is to shoot down helicopters, sometimes that's all I do. Often you see the same guys flying around and I learn how they respond to AA, and once I've learned their way of flying, I kill them until they stop flying.
I thought the main appeal of Battlefield was that it provides several ways to play, as opposed to infantry-only fps games; I don't support that they're building "games within the game" by making expansions especially for tank enthusiasts, or especially for close quarter fanatics. Battlefield to me is a large map, with portions which are great for all the types there are; it should be a sandbox in that regard more than be directed and focused like we're seeing indications of in BF3.
BF4 should be like this: Here is a huge map. Go play!
Putting c4 on jeeps then driving the jeep into the bad guys then jumping out of the jeep and blowing it up.
Then putting c4 on a jet then flying the jet into the bad guys and causing huge ass dumb explosions.
Can't tell you how many servers I've been banned from for lacing C4 on unmanned jetfighters and triggering the detonator when a teammate spawns in and takes off.
So fucking worth it.
I think I love everything - infantry, tanks, helicopters, jets, you name it. It's all fun. That's the best part of BF: if you get tired of one role you can just switch to another mid-game.
I tend to gravitate towards jets on maps that have them because achieving air superiority on your own and then proceeding to rain death from above like a freaking thundergod is such a power trip.
But it's all good. As for my favorite part of how BF3 differs from the rest, I think it's in the infantry role though: movement and guns just feel that much better compared to the previous games.
I've recently been playing BF3 Vanilla with a group of friends on P.C after a long, long absence from the game. They all own the D.L.C, and tell me it's quality, but not worth the asking price in retrospect.
Totally worth it if you bought Premium, not to mention it came down in price soon after release (PC version).
I didn't play Battlefield 3 for long. I really didn't enjoy the way they made the combat so much more fast-paced than the types of firefights you would get in BC2. Also, the game really suffered from a lack of personality. Everything felt too cold and sterile.
That said, I got a little enjoyment out of the multiplayer. It definitely didn't keep me hooked though.