Though I share many similar sentiments with other folks who played Duke Nukem Forever (two weapons, regenerating health), those are issues that could have been solved relatively easily (and in a way are on their way to being a little less irksome--four weapons instead of two. It's not the best solution, but it's the least we could have gotten to begin with).
My biggest issue of fault with DNF' is it can't seem to decide what pacing to employ. When I played Half-Life 2, I thought it was perfectly paced. By the time I was beginning to feel exhausted with one section or another, I'd reach a door that led to a new and interesting scenario.
In Duke Nukem Forever, everything goes by so fast, and by the time I was just beginning to become somewhat interested in a given section, they'd throw in a puzzle, or driving element, or I'd hear that background guitar noise signifying that the fun was over, and I'd slump to the next loading screen feeling rather unsatisfied.
Duke Nukem needs to be slightly less linear than DNF. It also needs to be slightly longer. Those are two things that require significant effort to combine, but if you want something to be good, you need to put in effort.
12 years is a long time, but what kind of effort was really put into the project? And not just work effort, but creative effort as well. Duke borrows so many things from other games, that I agree with Yahtzee on what he said: You can almost see the progression of game development throughout its history. The beginning of the game has a distinct Halo feel, moving unarmed through plot sequences until you get your first weapon and start defending yourself and your home. Then it goes through other phases, Call of Duty shoot'em up, an annoying driving section (note that I didn't find the driving as annoying as I did the being forced to stop driving every few minutes because "I ran out of gas".
I enjoy the concept of savoring a game, or savoring game time, and indeed I try to--but the game has to do some effort in convincing me that I need to take that much time. When I play Crysis, it's large open area lets me do things different every time I play it, and I can enjoy going slow and clever, or fast and furious; Duke Nukem Forever is too straight-forward to require much thinking, and while that's fine for an FPS game, it's not fine when it's more fast than furious. And DNF is really, really fast. I finished it in eight hours--sure it was the only thing I played for a day and a half, but while I was playing I was exploring the possibility of re-playing it, and I found very little to interest me.
There were also some moments in the game that might have been more entertaining to witness. A moment when a pig cop--which they can't even be called that anymore, now they're just mutated pigmen for the most part--closes a garage-style door on another pig cop made me laugh, as those little moments tend to make me. When I was walking slowly down the center path of a small classic western-style village, I saw tremendous opportunity for expansion. I saw pig cops playing five-card stud in a bar, giving an opportunity for another ego boost: if you kill three of them with a pistol and two with a shotgun, full house! Mow 'em all down with a Stinger, Straight Flush! And I was very, VERY disappointed when the pig cop that I saw slide into the street wasn't wearing a cowboy hat, hands at its sides ready to quick-draw its guns.
Small moments like that would have taken even more time, but the result would have been worth it. Despite taking 12 years to come out, Duke Nukem Forever feels rushed--again a point I agree with Yahtzee on--and ooooooh my god I didn't mean to type this much, so I'm just gonna stop here. I'm hungry, I enjoyed playing Duke Nukem Forever, albeit disappointed with the final product, and if they don't release the modding tools my disappointment will dissolve into abject rage and frustration. Seriously, when has modding EVER ruined a game--particularly a game that most would hail as not being very good in the first place.