I like the idea of the DM playing the monster if he/she wants to.

Adapting to the player's decisions is important so an intuitive system is needed to create situations as the need for them arises.

There should be limits, no one likes fighting an endless horde of enemies every few steps, but at the same time encounters shouldn't be too easy because of a small resource pool.

Permadeath/new character option( My DM does this if we mess around too much.)

Personally I prefer playing the character, but I convinced my DM to buy the game at release, I am really excited about this mode.