PC.IGN has posted some comments by 3D Realms' George Broussard, which they spotted over at 3D Realms' message board. There's two quotes where George directly addresses the fan concerns about the game...here's one of them, which explains why the game is taking so dang long:

Today’s games are MUCH more complex. In Duke 3D all the characters were sprites. Today they are polygonal models, have to be skinned, and then animated or motion captured. A VERY long, tedious and complex process. Also games today are "deeper" and require more than key/door stuff. That means more time to develop. Note all the so-so fps games that didn't sell well in the last 2 years and you will see that they were simple compared to more complex efforts like Half-Life. A game like DNF is probably 5-10 times more complex that Duke 3D was.