time
There aren't nearly enough hours in the day to get the things I would like to get done accomplished. Perhaps another reminder to be learned from graduate school: nearly everything takes far longer than you expected. The same could be said for many of the important events of life; if you don't budget for accidents or interruptions, you'll never arrive on time. Of course, it never seems to matter when I plan anything. Reality is resistant to this. On the other hand, having occasionally tried to fly by the seat of my pants, I find that having some form of plan is better than no plan at all. At least then you have something to complain about when everything falls apart.