I have GMed a lot and, although I have most control over the plot, I don't like having too much control and I try to find a ways to give RPers ways to influence the plot that could totally change how I had figured the RP would go. I view my RPs as collaborative experiences and, as Pachamac said, if I feel like I'm just writing to give RPers to react to, then I know I'm doing something wrong. I've been in RPs like this and didn't enjoy myself, so I try to avoid doing it myself.