The idea in D&D is that for the whole round you're sparring, dodging, feinting, parrying and have one likely chance (attack roll) to actually land a hit. If someone fails to take up the spar/dodge/feint/parry thing then your normally-parried blows wind up hitting them more often than normal.
In every version of D&D (since 1st), if you're in melee and run away, your opponent gets one free attack on your backside as you turn and run off. That's the origin of what is now "attacks of opportunity", applied to a bunch of other stuff.
If you wanted to streamline it you might just prohibit anything that now causes AOOs. Once in melee you cannot:
- Move.
- Cast spells.
- Fire ranged weapons.
- Attack unarmed.
- You can run away but take a free rear attack.