Can only the monster's "owner" play cards which affect the monster's roll (and what about enhancers and such?) Also, how well balanced is this for 2 players (with the card game you need at least three for any semblance of balance...)?
Cards that can affect die rolls will say things like "when you roll" or "when anyone rolls". Enhancers are handled exactly like in Munchkin, anyone can throw one on.
We playtested it a few times with only two players, and it wasn't as good as with 3 or 4, but it is playable with only two. With two players you can play the entire game cooperatively, but munchkins rarely want to share a win.