Hiya
IMHO, you should go back to thinking like a 1e/2e DM. In other words, your "job" as DM is to run a fair and neutral game where common-sense is the overriding factor for all rules adjudications.
In the case of AoO...keep using them, but apply generous amounts of common sense to each and every one. This will result in AoO's that "make sense" happening, or at least having the potential to happen, while AoO's that "make no sense at all" will simply go *poof*. For example, a fighter is engaged in combat with an ogre. Suddenly, a kobold goes running behind him heading towards a door. As per the AoO, the fighter would get an AoO on the kobold who just ran behind him on his way to the door. However, this would entail the fighter turning 180 degrees, swinging at the kobold, then turning back 180 degrees...while the Ogre would apparently just stand there, waiting for the fighter to do his thing. Silliness I tell you! When you apply common sense to the situation, it changes. Suddenly the fighter *can* turn and swing at the kobold, but the DM should rule that the Ogre then gets an AoO against the back of the fighter. If the fighter player deems it worth it, he can, or he can let the kobold go and keep his focus on the ogre.
So, just apply common sense and have fun!
^_^
Paul L. Ming