The “attack of opportunity” (AOO) rules in Wizards D&D 3e get disparaged a lot. The rules—at least in the 3.0 PHB—weren’t described well, but the rules themselves are actually pretty good.
Nigh every wargame of a certain level has “zone of control” (ZOC) rules. These are rules that attempt to compensate for the turned-based nature and other artifacts of the game. If you don’t realize that the AOO rules are ZOC rules, then they can often not make a lot of sense. Like much of the combat system, they’re an abstraction, so they can’t be taken too literally.
AOO have a couple of advantages as ZOC rules. One, they are actually pretty simple in play for their “payoff” compared to other ZOC rules. Secondly, they are not as “hard”. A character willing to take a chance at being hit, can effectively ignore the AOO. Or one PC can draw an AOO from an opponent preventing that opponent from getting an AOO against another PC later that round. (Provided the opponent lacks the Combat Reflexes Feat, of course.) Some ZOC rules never allow for circumventing them.
Personally, while I like the AOO rules, I don’t think they are appropriate as default rules for the flagship role-playing game. For a skirmish wargame or for an optional set of advanced combat rules for a role-playing game, sure. It’s the context I don’t like about them more than the actual rules themselves.