Actually, one of the reasons it's taking so long is that they keep switching out the tech for new, better ****. DNF is probably now on an engine at least as polished as Doom 3's, which is to say, okay but not next-gen by any means. Who knows, they could trot out an engine that's actually impressive.
I remember seeing the old screenies that showed a graphical sophistication not beyond the original Half-Life. They were shown around the time of the "FPS war" that came after the release of Quake 2, the one that Half-Life ended up winning. IIRC, DNF was supposed to compete with the original SiN (heh) Unreal and Half-Life during that time. Never happened.
Its place in development hell aside, the fact that they've switched out engines so often obviously doesn't bode well for the game itself. It would seem to indicate that visual quality is high enough on their list of priorities that they would scrap the game and start over when a certain level of sophistication isn't attained, and that's never good. Is the game such a ****ty play that they require a "hey, that looks cool" to justify the game's development?