Strengthening the attack of opportunity would go a long way towards allowing melee characters to control the battlefield.
There's a long story about how Fighting Men used to get attacks against monsters that turned there back on them, and how in d20 such attacks became really weak because of the way that physical damage scales (spread over multiple additional attacks instead of advancing a single attack), and because of the 5-foot step and concentration checks to cast spells in melee, but the end result is that its hard to keep enemies from moving past you if they really want to without a boat load of feats (that only marginally improve the situation...why do Stand Still and Pin Down blank your damage?).
