The big one for those of us with an AT&T unlimited data plan is tethering. They had to grandfather our plans in because they sold them to us as unlimited, but they will not offer tethering as they are trying to force all users off of those plans and on to the 5GB plan.
They certainly did on my iPhone 4 (as recently as 6 months ago). I haven't tried it on my iPhone 5, though, because I haven't jailbroken it yet. I guess I'll find out soon enough.
