At first I really thought core needed some kind of OA rule to make melee stickier. But eventually I came around to the playtest's way of doing it. I think of it like this:
OA rules do very little for the DM. If the DM doesn't want the kobolds to rush past the fighter to gank the wizard, then they don't. DMs don't really need to use combat rules to justify how they run monsters, particularly when those rules are really an abstraction anyway.
And in practice, OA rules don't affect the players that much either. Low hp classes already have incentive to stay away from melee. Classes that want to duck in and out of melee tend to find ways to do it even when OA rules are present (high AC, spring attack, etc...), usually by exploiting even more rules. Do away with the whole mess and player behavior doesn't really wind up that different in the end. About the only major change I saw was with the Wizard ducking in and out with shocking grasp, and that's more a problem of the Wizard not having a comparably damaging ranged at-will.
I agree with you on the issue of AoO. At first I thought it was missing, but then when we played, we saw that combats were becoming more dynamic and interesting - more movement options and choices. There are still plenty of ways to play tactically without AoO.
Shield wall/block at choke points
Have defender near a squishy or injured PC
Ready an action to hack at the monster that runs to get the Wizard
Wizard runs away or finds cover
Grapple the monster that is drooling as it stares at the spellcaster or injured pc
Wizard can use frost ray vs. the same drooling monster
Use ranged weapons to attack the wizard who ran up, used shocking grasp and ran back.
Set up obstacles to make it harder for monsters to run to the back ranks (tip over tables, etc.)
That being said, WotC does need an optional rule for AoO so that people who really want it can use it. At the very least, perhaps there should be a way to "disengage" from combat. Maybe a skill contest.
Overall, if we play without AoO, we have to assume that all adventurers and monsters have enough combat training to know when and how to move through threatened areas.