I've read through the playtest document and I like what I see. Sure there are some obvious picks and no-picks as far as mythic abilities and feats and so on, but I reckon that'll get ironed out. Mostly.
What I'm concerned with, and will bear in mind as I playtest, is that it appears to require a crap-ton more bookkeeping, both on the players' behalf and the DM's. Everyone has a ki pool now. Feats need to be cross-referenced between mythic and vanilla and the new prereq of mythic tiers has entered the picture.
If that doesn't faze the average player, then they have to reckon with the lesser trials. Much of the wording appears to require the DM to keep track of the success of the trials, especially the "do X to a creature with full hit points" trials. How should this be handled? Does it not break immersion when, after a fight, the fighter raises his hand and asks "so, was that ogre at full hit points, or..?"
I'm not complaining, nor have I tested it in-game. It's just a thought I've had, and I could be totally off base.