I think attacks of opportunity are stupid. I've never liked them and I have never looked at a feat or class ability or trait that added a benefit to attacks of opportunity and remotely felt an interest in taking it.
One of the things about it is the illogical nature of it in regards to the rules. You can make one attack of opportunity per round. Why the limit on it? Time constraints? That makes no sense. If a Fighter can somehow fit four attacks into a six second period of time, plus one or more from his off hand weapon, plus an additional possible attack of opportunity that may or may not come up, why can't he fit in as many opportunity attacks as regular attacks? Combat Reflexes allows you to make attacks of opportunity up to your Dexterity modifier because you have good reflexes or something. If you have such good reflexes, why not a bonus to Reflex saves or a Dodge bonus to AC? If you are so agile and such that you can fit more opportunity attacks in a six second period than a normal person, why can't you just add in an extra regular attack? It's ridiculously abstract. I'm really quick and agile, so I can make more attacks in a six second period than most people, but only if you trip over your shoe laces or charge at me swinging your massive sword or attempt to disarm me or attempt to sunder my shield with your hammer. This is one of my constant gripes about Pathfinder/DnD, the combat and such is so abstract and inconsistent with its rules logic that if I think about it longer than five minutes, I start to stroke out and the system starts to break down.
I've thought about removing them from the game, but we barely use them ever so it feels like a pointless gesture. If anyone ever charged or disarmed or feinted in combat, I'd consider it, but no one does. I've considered treating them the same way I do critical hit confirmations. We don't use them, but if a feat helps them and it is a requirement for another feat or prestige class or something, you have to take the useless feat. Sorry. I don't know if that system is overly punishing, but the feat that offers a +4 bonus to confirmation rolls on critical hits unlocks the whole chain of critical feats. The ones that allow you to blind and stun and bleed on critical hits. I mean, with a rapier and Improved Critical and a one or two of those feats you can do some serious damage.
The other option is to figure out something to replace them with. So you provoke an attack of opportunity when you do something that would in theory leave you more open to attack. When you charge a guy, he gets an attack of opportunity. If you leave an enemy's threatened square, you provoke one. Any time you perform a combat maneuver, cast a spell, fire a bow/gun while threatened by a melee weapon, use unarmed attacks against an armed opponent (without Improved Unarmed Strike that is), use a skill, or drink a potion, you provoke an attack of opportunity. There's a whole table of actions that provoke them and what I've listed is not comprehensive.
So what would I replace them with if I were going to replace them? Off the top of my head, flat-footed. So flat-footed AC represents you being off your footing or otherwise unable to defend yourself by serpentining and hopping around like a ninja and such. You're denied your Dexterity bonus to AC because you're not aware of your attacker or you're immobilized or something along those lines. So why not say that any time you would provoke an attack of opportunity from someone, their attacks against you use flat-footed AC for the rest of the round? It is a short enough period of time that it doesn't leave you super vulnerable, and it doesn't slow down combat by adding in extra attacks, but it does show that you are more vulnerable than you were.
Granted, it doesn't slim down the rules at all. It's just an alternate take on an already in place system. Plus, you have to figure out what all the attack of opportunity feats do now because if you're going to change a mechanic instead of just removing it, you have to change the rest of the game with it. I dunnno, whatever.