Tried the demo, and honestly all the complaining about it is really unfounded.
Ok sure, theres regenerating health, but one, its not just plain regenerating health. Every time Duke's ego goes red, its knocked down a peg when it regenerates. And duke can do certain ego-boosting things to regain the ego that was lost (but this feature isn't really shown in the demo at all, hence all the complaining). Its not just normal COD-like health, it actually is innovative and when we get into the harder levels, we'll actually notice that. (now that I think about it, Ninja Gaiden II did almost the same thing with health)
Two weapons was a design choice, and theres a thread about it here somewhere. However, my opinion on it is 3d realms probably did it because theres over 20 weapons or so in the game, and that would be hard to manage on a controller. It worked with half-life 2 to have multiple weapons because there were only 10 or so.
Thirdly it FEELS like Duke Nukem. Especially in the beginning of the demo. The second 'desert' part not so much, but thats because its a small section from the middle of the game and we have little to no context on what duke is doing out there. But the humor is there, the tongue-in-cheek little things duke can mess around with in the game world are there, and all the same weapons from 3-d are there (the shrink ray is great). Theres even non-battle sections (puzzles, etc), how often do we see those in shooters now-a-days?
Fourthly, graphics look fine to me.
Yes, the demo is "lackluster", its 'missing something'. Its missing THE REST OF THE GAME. When I actually do play the rest of it, I'm probably going to come out of it thinking its a great game, because its goddamn Duke Nukem. Also, some people are complaining the demo was short. Well if Gearbox is true to their word, the game should clock in around 16-20 hours of gameplay depending on how fast you breeze through it.
Point is, Always bet on Duke.