I chose PC and NPC characterization and relationship, but I have to say that any really good campaign must draw from a bit of all of the given choices. I strive to make the PCs feel like they're really in the campaign world.
I also like to bring out a few D&D stereotypes like good ole' dungeon crawls and a few pointless random encounters (not necessarily combat based brw). That kind of thing is essential in all of the extended campaigns I've run, just b/c I use those kinds of devices to help break up some tension and also give myself a bit of fun.
C.I.D.