Close attacks provoke also, IIRC, but that's not the point.
As others have said, we're both wrong (it's area attacks, not close attacks, that are the other case) - but that's peripheral, really.
Other elements modify your AC vs. OAs, and I'm sure there's more than one magic items which enters the picture, too.
Sure, but that applies to any attack, surely? If you object to it, that's fine, but it's not really an objection to OAs.
To be clear: I'm using OAs in my game and think of them as a fitting part for 4e's tactical combat system. They do work, indeed, but the possible interaction of other modular elements - you gave conditions and a class feature - is something which works better in theory than in practice, IMHO.
On the other hand, I find this a supremely easy way to learn rules. I learn how all the individual wheels work, and I find out how they interact through experience. I don't
need to remember all the stuff about how they interact, because they just
do. I find this makes the system a lot easier to remember than piling up a slew of case law around a fluffy and imprecise guideline. Maybe this is a fundamental difference in the way humans think? That might explain the deep divide I see between those who want a clear system that is complete within itself, and those who want freedom for the GM to make "rulings not rules"?
No.
Let's throw opportunity attacks under the wheels and let them spin out for a while.
I'm not at all sure I know what this really means (ditch them? see how they flow when moving fast?), but without them you need something to stop rather crazy combat mobility. Being given a "free attack" if an enemy moves away without spending an action is just an OA dressed up in a fashionable outfit, but what else do you suggest?