The game certainly isn't terrible. I think most people are overreacting just because the game has had a long and troubled development period and they were simply expecting more.
That being said, it's certainly not great. There were some parts that I enjoyed and some parts that make me want to scream. It sort of seems like half of the game was designed with regenerating health in mind while the other half of the game was developed around an old school medikit system. There's many parts that you can tell were ripped straight out of 2001 and other parts that are decisively more modern.
The middle segment of the game is probably the best and most solid segment and feels like the most modern. The rest of it is hard to place. Also, it has physics puzzles. That's the last thing I'd picture a Duke game having.
The weapons are, by and large, underwhelming. It has many of the Duke Nukem 3D staples -- such as the Devastator, Ripper, Freeze Ray, and Shrink Gun -- but thanks to the harsh ammo restrictions and the ability to only hold two weapons, I find myself switching between the pistol, shotgun, and ripper throughout most of the game, since they hold the most ammo and generally do the most damage.
Boss fights are hit and miss. Some of the bosses -- such as the Battlelord -- were clearly designed around the player having a personal arsenal. You see, bosses in DNF require you to hit them with either turrets or explosives. Turrets are fine, since the worst thing that can happen is that they'll overheat. That being said, you typically have to dispatch bosses with an RPG. An RPG that holds five rounds, that you have to repeatedly refill in the middle of a boss fight. It works fine in the levels during that golden middle segment, but outside of that? Well, imagine facing a Cyberdemon in Doom. Picture that your rocket launcher only holds five rounds. Now imagine getting off five shots, then having to run to one or two points to restock on ammo. Now envision the Cyberdemon busting out a hitscan weapon that does a fuckload of damage if you stay out of cover for too long ("too long" being mere seconds). If it doesn't sound pleasant, that's because it isn't.
Duke also doesn't talk enough. For a character that's so beloved and developed, he stays silent a lot. He pretty much gets bitched at by the president for his antics in DN3D for the entire game and doesn't respond once. He doesn't interact with the other NPCs too much at all, only with the player. It just makes him seem strangely detached from the world. With the gameplay style that 3D Realms and Triptych decided upon, it's almost like playing a Halo knockoff with a Duke Nukem voiceover.
There are some parts that really bring you into the Duke Nukem 3D mindset. The intro and beginning level are very Duke. The strip club is another section that just screams that this is a Duke Nukem game. High levels of interactivity, plenty of one-liners, and chock full of ego. It's a shame that they stuffed everything that is Duke into a few levels and didn't spread it out over the course of the game. Throughout the majority of the game, the only thing that really reminds you that you're playing a Duke Nukem game is the occasional one-liner.
Overall, I thought it was average. Some people are really, really overreacting (some people actually think that DNF is worse than Daikatana. I mean...what?!) and a smaller group are really underreacting (PC Gamer US's overwhelmingly positive review certainly is not deserved). It's a Halo clone starring Duke Nukem.
That being said, the released product is still 3D Realm's game. Now that Gearbox owns Duke, I'm curious to see what, if any, future titles are in the pipeline.