We're excited to announce that we've fully migrated the Xbox Forums to their new home, here on the Microsoft Community forums!
As of today, going to forums.xbox.com will redirect you here to the Xbox area of the Community forums. We've moved all of your existing threads over so you can continue and start conversations with the new forums experience.
We sincerely appreciate the feedback you have provided over this transition and can't wait for you to check out the new Xbox Forums!
The reason that you can't, for example, with BF3 is that the PC version has much more freedom with servers and modification, and though I'd say that gap is closing very slowly, it'll be awhile before we see more cross-platform play.