The rules show some great stuff coming. I like the way they are integrating this expansion in ways to make it backwards compatible with the previous sets. The addition of "variant monsters" of existing creatures from prior sets is a cool and easily implemented idea and the wandering monster substitution rule helps to insure that you'll always have the chance to use vermin in your games.
I'm not too keen on the stamina monsters as I don't like the trend it may start. The beauty of this game is its simplicity and adding monsters that you have to track damage with as well may start to drag out the game if it starts becoming the norm. The addition of only two of these types of monsters is likely ok for now. It's interesting that some of these mid-level bosses have more life than the catacomb lords themselves. Gotta love that devoured power though.