Taunt an enemy player and they are forced to target you and turn and face you. "Taunts" like death grip and distracting shot would do the same but the enemy player would be unable to change targets for the duration of the taunt. Perhaps some sort of tunnel vision graphic effect to go with that.
Diminishing returns are already in place for PVE, but it would make sense to shorten those for PVP to avoid taunts becoming really annoying like death grip is already with multiple DKs.
Would provide a niche for "tanking" in PVP. Has the potential to be very interesting, but also would shift the balance of PVP in unforeseeable ways. For starters taunts would become interrupts of a sort. Find people auto-running and taunt to alter their course off a bridge or cliff (wonder if that works with death grip presently).
As a tank or class with a taunt or a pet that can taunt I find the idea really exciting. Just thought of pet taunts... hunters would get a ton of PVP utility. Having your pet take a player off you if only for a second would provide an advantage in many situations. Suppose the same goes for warlocks.
Edit: Plus this would promote team work. A taunt is useless 1v1 or 1vn, everyone would be after you anyway. With the exception of pets of course. This will allow people to help out their healers or perhaps a squishier DPS allowing more diversity in PVP builds depending on who you are grouped with.
Something to think about.