I've been a Duke Nukem fan all my life. Hell, it was one of the very first games I have the memory of playing (I remember playing 1, 2 and 3D around the same time).
Yet, even still, I really don't have any qualms about the demo. It had the humor, over-the-top action, personality and interactivity of a Duke game, and that's really all I need.
As for all the roasting of Gearbox going on, it's like many before have said: Gearbox simply polished up and finish and already mostly-done 3D Realms/Tryptech game. If you really don't like the direction the game took, they're the ones who definitely hold most of the blame.
Also, from all the things I've read, the reason that Gearbox didn't just rebuild DNF from the ground up is simply because they wanted to release the game as 3D Realms had intended, not try their hand at making a whole new game that might not have been in the spirits of 3DR's original plans.