It's funny reading all these threads and the number of people who have connected 3DRealms and over-hyping. I have never heard/read any example of overhyping by 3DRealms re: Duke Nukem Forever. In fact they have said almost nothing about the game.
Their only crime has been to announce it early and then release a trailer.
I suppose you're referring to the E3 trailer of 1998, but you're leaving out a few other releases there: early screenshots in 1997/98, a big preview in PC Gamer late 1999 (of which the screenshots were also released online), and then (most notably) a new movie trailer at E3 2001: http://www.3dgamers.com/games/duke4ever/
I'm not saying 3D Realms over-hyped DNF, certainly not after 2001, but they talked about it in the first few years of the game's development and released early media, and that's what people remember.
I guess they simply were a late adopter of the Valve principle.