To me, opportunity attacks serve an incredibly important purpose in the game.
1. They prevent people from just walking around the melee folk/front line
2. They prevent people from using ranged weapons in melee
3. They prevent people from casting spells in melee
4. They make it hard to run away from someone if you get caught in melee
While AOOs are not necessarily the only way to serve these goals, I haven't heard any other better suggestions in this thread and I don't see too many inherent problems with the system itself to justify swapping it out for (in my eyes) equally flawed systems. My rebuttals of a lot of the points brought up against AOO's follow:
First of all, opportunity attacks DO prevent folks from casting or shooting in melee, even with 5 ft. steps. First off, the threatening warriors might have reach - in which case there's no simple escape. Second, multiple threatened areas can overlap or cover large ground, so a party attacked on two sides might threaten a mage even if they were to 5 ft. step "backwards".
Even if we ignore these points, there's a simple solution to ensure that the rule functions as intended: say that casting spells and shooting arrows begins at the start of a character's round. As a result, mages and archers would provoke AOOs before they can 5 ft. step if they cast or shoot arrows. This is the way I run it and it works great. I also make the concentration and Defensive casting DCs higher than normal - it makes casting in combat quite risky compared to defending and trying to move carefully towards allies. Moving in to fight someone (charge or otherwise) never provokes from the targeted enemy.
Second, I don't know how other people are having OAs slow combat down so much. It's very simple for my table of 4-5 players and runs quickly. Someone declares their actions, you point out the OAs and from whom (and if there's an obvious alternate route point it out and let them respond), roll all the dice, deal out damage, mark it, and move back to the players turn. Shouldn't take longer than a minute tops to handle all OAs in a combat round.
Third, a player's ability to understand a rule is only relevant if the rule is overly convoluted or poorly written. OA rules need not be either. How is it difficult to narrate an AOO? Why is a player having difficulty understanding the rules and why isn't the group helping them understand/offering alternatives?
my simple AOO rules:
If you do any of these in a threatened area, you provoke an AOO
leave a threatened area without disengaging (if disengaging is an option)
Run through a threatened area without attacking the threatening creature.
drink a potion
stand up
declare a spell
declare an attack with a ranged weapon
you never provoke from movement if you move 5 ft. or less in a round. Casters and archers must declare their spell casting/shooting at the start of their round if they are threatened because that is when these processes begin. Otherwise, they cannot take those actions.
What is difficult about this? I'm honestly curious where people are getting hung up.
Here's my list of things my thoughts for an AOO replacement mechanic's qualities:
1. It cannot simply say "NO" to a player or NPC. They should be able to try to get around someone, even if there are huge consequences that make it a bad option.
2. It cannot be class-exclusive. There are a lots of melee classes in the game and they all have a chance to contribute to holding the line, in my eyes.
If AOOs or some other mechanic for protecting areas of a combat area to protect the casters/others isn't in 5e, then that doesn't bode well for the system in my mind and it would seriously influence my decision to adopt 5e. I especially don't want AOOs to be a Fighter-only thing. In my games, Barbarians, Paladins, Rangers, and Rogues all make pretty heavy use of it. If another mechanic replaces it, it needs to serve the same purpose. Putting OA's in a module is fine as long as the module is well-balanced and well written. Streamlining the actual expression of the rules is fine - but we can't lose functionality just because we're concerned that some people have difficulty understanding it the first time they pick up the book (I've almost never understood half the rules I've read in any game I've ever played the first time I read the book).