Typically the players, in my experience. I'm a fan of pulp action-adventure and 4E D&D so I present set-piece encounters that are dynamic with multiple types of enemies, alternate win conditions, areas filled with stuff that can potentially be used, environmental factors, etc. I have the monsters do all the cool stuff I wish my players would do. Swing from chandeliers, throw bowls of hot soup, sand in eyes, etc. And still the players simply walk up to an enemy and pound on it until it's dead, then move on to the next one. You can lead a horse to water but you can't make him drink.