“…Your players, for the most part, are not nearly as interested in your complex plots and elaborate setups as you are.”
I couldn’t agree more — and this is something a lot of GMs overlook (at times, myself included). This tidbit comes a close second (substitute “any RPG” for “D&D”):
“The tragedy of D&D is that sometimes we confuse time and effort with interest or think that the amount of time and effort we spend somehow earns us a certain degree of interest.”
Other advice follows — including, interestingly enough, an explicit reference to hashing out a social contract  to address these issues (and others). That’s something D&D, and RPGs in general, should do more often.