Room after room of humanoid monsters with precious little to differentiate them other than a handful of hit points.
"But it's up to the DM to differentiate them in how he describes and presents them to the players," I hear you cry. "For room after room after room after room."
As a DM I was left cold by this playtest. The prospect of running the game is not an exciting one.
1) Playtests can be boring, as the designers are looking to test a specific set of conditions (in this case, I believe, how the classes interact with each other and the setting) which necessitates controls that may well not be ideally fun (such as samey boring monsters).
2) That said, this is a valid criticism that should be conveyed to the designers a la: "These monsters seem like boring bags of hitpoints as opposed to the dynamic monsters I really like in x eddition which do y much better than these!" The whole point of the playtest is to hopefully improve the mechanics for both playability and fun!