Scheduling, tied with time. I need to strike while the iron is hot.

If I perceive a player that isn’t engaging, I get tied up internally trying to figure out how to engage him/her. It becomes a quest.

Players that refuse to take the reigns of the fiction expecting me to spoon feed them. For this reason, I am trying more GM-less games.

HUGE for me: while I enjoy some game prep: tinkering with rules, wondering how this or that would spin out, engrossing setting history, a bit of straight up day dreaming; I get so tied up in it that a bit of my excitement fizzles as what was fun is now work; hence, a preference for zero to low preparation. This ties in solidly with scheduling for me. I must play before my imagination kills the fun.