EDIT: I have decided to run a D&D game set in the rather sketchy world hinted at in the PHB and supported in most modules. Please see Documents->Darkwylde in the navigation menu for rules and guidelines.
As I inch closer toward running my first 4th Ed. D&D campaign (just D&D, hereafter), I've been giving a lot of thought to the source material. My strength as a GM is in roleplaying NPCs and world building, or so I flatter myself. This has led me to run primarily my own worlds. Over the last several years I've drifted away from that and run Arcana Evolved and Forgotten Realms, using their worlds with original content of my own devising. Now I'm at the point where I want to write a fantasy novel, possibly with a D&D tie-in, and making my own world for that effort has a lot of good synergies.