Another restriction nobody has noted is that you can't visit the same destination twice.

This seems altogether too restrictive. There is a very limited amount of the A3 network available - mostly the shorter flights and no "prime" routes. I suspect you can do better, or very similar just booking the flights in most cases - especially with the 20 day requirement.
There are a limited number of seats available for the passes but exactly how many is not made clear and you wouldn't be able to tell until after purchase. If it's just the regular award-seat pool then... good luck!
Round-trip only (I almost never do round trips as I like to take in multiple destinations).
Weird blackout dates - I understand high summer, Easter etc. but what happens at the end of November or 21st to 26th March, for example - Easter is mid-April this year.

There is no mention of booking class and what miles will be earned. There is a catch-all in the T&C that says "Otherwise, the terms and conditions of the AEGEAN Pass apply." but do we REALLY know these will earn 100% mileage - or any at all?

All in all, totally not worth it for me. I don't see this one being very popular at all.