Some things you can expect: Even the big maps in Halo aren't even close to a big BF3 map. So that will take some getting used to. 4v4 maps are somewhat comparable though. The game is faster-paced, has more of an arena shooter feel, but not crazy twitchiness like CoD. The guy who talked about the learning curve is right. Just hang with it for a little while and you'll get pretty good.
Welcome to halo, TC. I also enjoy BF3, heck I'm even playing it right now on my 360. Like others said though, you should be able to enjoy yourself just fine. The gameplay is slower paced compared to BF3 but by playing enough, getting used to it and with the learning curve, you will be all right.