Yeah, those elements certainly are ripe for improvement. Thankfully, it sounds like two out of the three problem areas you mentioned should be improved in 13.
I'm not sure that we'll be able to slide tackle whenever we want, but supposedly interceptions is vastly improved. It won't give you power to slide to intercept, I think, but hopefully it'll upgrade that area of the game to not be such a noticeable flaw.
Jockeying is a bit of mess in 12 but thankfully that'll be much better in 13 hopefully with the combination of lateral contain and the improved tugging system, or whatever it's called. Though I doubt the improvements will be enough to fully satisfy a lot of us here, the early impressions have been positive.
Heading... yeah, it's not good now and I think you'll see it get addressed with either FIFA 14 or 15. That area of the game is being brought up more and more, and I can't help but think it's gotta be moving up the devs' priority lists.