I think what sticks out to me the most, and what I've wondered myself, is how they intend to handle synergies with the rotation.
Blizzard stated that their approach to synergies was to release 0-2 cards per set in any given tribe or archetype to try and ensure it wasn't overpowered. Their argument was that over the years, this meant that they'd eventually print enough cards to ensure that the archetype worked, but never risk making it a completely over the top deck by printing, say, four way too powerful cards at once.
The only exceptions they made to this were if they decided they wanted a major theme for an expansion, at which point that theme would get a lot of cards, but nothing else would (see Dragons in Black Rock or C'Thun in Old Gods).
But with set rotations, this means that these slow-but-iterative tribes and synergies can never actually get going, because as soon as they start working, their critical cards are torn away from them. Since Blizzard doesn't rotate anything into the core set, we just have a bunch of mediocre to useless synergies, or ones that are incredibly transient. We look set to lose our most successful synergy (Dragons) in the near future as well, and a big part of why that worked was that there were strong cards in the base set that didn't keep falling out.
