Attacks of opportunity need to be dropped when 4th Edition comes around.
There are easier, better, more streamlined ways of making concentration-intensive or reckless actions interruptible or dangerous in melee. And AoOs just don't make much sense, logically speaking (why get more attacks per round just because someone else's being reckless?).
Here's something I came up with around a month or two ago, for use in one of my campaigns (soon as I get the chance to start up another campaign, anyway). I'm sure the people at D&D R&D can come up with something at least as simple and sensible, if not better. And anyway, I made this mechanic more penalizing for reckless characters, which probably wouldn't really be done in 4E though.
"Replacement For Attacks Of Opportunity
Sometimes in combat, a character or creature will have to drop his or her guard for several seconds to perform an action that requires most of his or her attention and effort, such as carefully disarming a trap, picking a lock, casting a spell, or initiating a psychic power. This leaves the individual open to attack with little or no chance of avoiding harm, which could disrupt whatever task they are focusing on as well. When this sort of circumstance occurs, it is called 'dropping your guard', and the descriptions for certain actions will specify if they require the character or creature taking those actions to drop their guard.
Any time an individual drops his or her guard, he or she is considered 'unguarded' until the end of his or her turn. An unguarded individual is effectively flat-footed, and thus denied any Dexterity bonus to Armor Class, as well as being denied any dodge bonuses and haste bonuses to AC. Any damage taken by that individual while unguarded could ruin his or her intense concentration, and thus interrupt his or her action. Any time an individual is damaged while unguarded, he or she must attempt a Concentration skill check to maintain his or her focus, and failing the check means that the damaging effect has interrupted his or her action. This works as detailed in the Concentration skill's description.
Whenever a character or creature drops his or her guard, any other individuals who threaten that character or creature may attempt to attack the unguarded individual once, even if it is not yet their turn in the round's Initiative order. Any individual wielding a melee weapon or a natural weapon threatens the area around them, within that weapon's reach, but unarmed strikes and ranged weapons normally do not threaten an area. Only certain feats, skills, or special abilities may allow unarmed strikes or ranged weapons to threaten an area. When a character or creature drops his or her guard, opponents threatening him or her may attempt to attack that unguarded individual if those opponents have not already taken all of their allowed actions for the round. Opponents who have already taken their allowed number of standard actions or full-attack actions this round, or who are not allowed to take any such actions this round, also cannot attack unguarded individuals in this manner.
Attacking in this way counts towards the opponent's allowed number of attacks for the round, and thus requires them to expend a standard action or full-attack action on their turn, as appropriate, reducing the number of attacks from that action appropriately. The opponent cannot attack an unguarded individual in this way if the opponent would not be able to make a sufficient number of attacks this round. Also, the opponent cannot use any special standard action attacks or special full-attack actions this round if utilizing this opportunity. Normal standard action attacks and normal full-attack actions are allowed, such as bull rush attempts, disarm attempts, trip attempts, grapple attempts, two-weapon fighting, fighting defensively, and so on and so forth, but not charge attacks, since the opponent will have already attacked this round before getting the chance to charge. Any bull rush attempt made when attacking an unguarded character does not allow the opponent to move with the unguarded character he or she has bull rushed."
