If, in RT, you enter the melee fighter's "area of attack" (ie: he's using a sword with an attack range of 3, or a polearm with a range of 6-7) you will take damage, then I imagine that would be a workable system.
you'll only be attacked if you attempt to leave the area of attack. hence the lock-down effect.
I think what I was aiming at in my post was the idea that
entering melee should be equally as dangerous as
leaving melee range with a fighter. Getting close to someone proficient in polearms should be hard, as they should be able to keep you at a distance while using the length of their (hyper) weapon to wear you down, for example.
This lock down system sounds good for making melee more risky for non-tank characters, though.
Edit; There is also the sense that melee range should be important. A long melee weapon (polearm, greatsword, flail [due to the swing reach of a ball/chain weapon], or such) should be able to demonstrate an effective range, getting close to the character should be hard because you'll take more damage at a certain distance, but once you close quarters with a close range weapon (a stabbing sword, such as a short sword) your close-range damage should outweigh their long range damage.
Thus: At long melee range, your defence / parrying vs a polearm would be important. If your character can parry a polearm attack at long range, then you'd presumably be able to enter their close-melee range where your weapon (if a short sword) becomes effective.
An attack of opportunity would theoretically be the attack an opponent would get when you move into or out of their effective range in such a system?
Edit#2: So, building on what
Tigranes said, and what
Zed said, a long-range melee character, with a polearm or greatsword, for example, should "lock you down" in their effective melee range, where a competent short-range fighter, with a falchion, mace, shortsword (effective short range melee weapons) could close into their effective range if they are able to defend/parry the attacks of the opponent. This would encourage interesting ways of building your melee character. The distance vs close quarters melee fighter. It would also encourage interesting ways of getting through or out of an opponent's lockdown, a rogue for example might be able to use a "trickery" skill to throw sand in the opponent's face to blind them, which could "turn off" the lock down effect for x simulated turns.