I am a GM for my group 90% of the time, so my issues are mostly from a campaign running and preparation perspective.
1) I would really like monster stats to be closer to that of PCs, and from the look of the playtest bestiary, they are not. They don't have to be exactly the same, but at least the same calculations for AC, HP, and attack bonuses would be good. I just think it really adds to the immersion. I'm aware that we don't really have proper monster creation rules yet, but this is my concern so far.
2) The check DC table feels too abstract for me. I can't really imagine what a task of a certain level is supposed to mean, unlike with 1E, where I could base the DC on the information I have about the task (lock quality for doors, surface type, for example).
3) Exploration mode rules seem very unnecessary to me. I've always run non-combat and exploration situations as a rules-light and natural process. I didn't even really use the exploration mode rules while running the playtest.
These are my biggest issues, but I do want to say that I really appreciate that 2E makes it much easier for me to run and prepare games.