I chose Dungeon Design, but for me I meant that to include the why something it there, DC's, traps, placing creatures, etc. And since not all of my campaigns are actually in a Dungeon I meant for it to include above ground adventure obstacles/challenges.
I see a large number of people are saying the NPC's are the longest time. My campaign is low-level at this time so it may be awhile before I come to realize how much of a time sink it may be in the future. For the most part now, I don't stat out NPC's fully *unless* he or she is central to the plot. And even in some of those cases the statting out is pretty loose. So much of the information goes unused I only stat out the basics. If the NPC is a spellcaster I do plot out his or her spells so as not to be tempted to let them have unfettered access to certain spells depending on what the party does (i.e. keep me honest).
Again, this may change as my campaigns reach a higer level where there are more feats, magical items and such to take into account in order to challenge the party.