There are many kinds of FPS shooters with many of them being all serious. Duke Nukem 3D inverts all of that by giving you over the top weapons, an Sci-Fi plot and a lot of references to pop-culture along side mature humor and funny one liners. Duke Nukem 3D does alot of things that many other FPS's don't do and were about to find out why.
On the surface, Duke Nukem 3D looks like your average Doom clone. However, the similarities stop right here as we get to the weapons. Duke Nukem 3D offers a pistol (though it now requires reloading and it fires a couple of bullets at a time), shotgun, chaingun and RPG just like Doom, However Duke Nukem 3D also has unique weapons to set it apart such as pipe bombs which detonate when you want them too, a shrinkray which can shrink enemies down to toy size for a few seconds where you can then squish them, a freezer which can turn your enemies into ice for a few seconds also while doing alot of damage, laser trip mines that you can place on walls, and finally the Devastator, which fires alot of missiles at your enemies and is great for taking out a bunch of aliens. When you gib an enemy with any of the explosive weapons, Duke will probably utter one of his one liners which never gets old. (Though you can turn that off if you find it annoying.)
There's also platform jumping in certain spots and you have the ability to jump and crouch down and while you collect key cards like in Doom, you also have a bunch of powerups that you can pickup. Unlike the powerups in Doom, you don't use them immediately when you pick them up (minus the normal health bottles and armor pickups), but instead store them in your inventory so that you can choose when to use them and the powerups and pickups are pretty cool. Along with the standard health and armor pickups, you got a medkit which carries 100% worth health to heal yourself, atoms that make your health go beyond the 100% limit up to 200%, steroids which give you a brief speed boost, nightvision goggles to see in the dark, Holodukes to display a hologram of yourself to distract the aliens, and of course, the jetpack, which is perhaps the coolest and best pickup ever in a video game, and let's not forget the scuba kit which allows Duke to breathe underwater.
The environment is very interactive with mirrors you can use to admire yourself, pool tables to play, destructible objects and glass, lightswitches you can turn on and off and of course, paying strippers to take their tops off thus showing you the goods. A lot of the interactivity helps make Duke Nukem 3D stand out from all of the other FPS's and combined with the weapons, the enemies, and the awesome levels help make Duke Nukem 3D the best FPS of all time.
By default, you can play Duke Nukem 3D the same way you can play Doom. There's a catch, though, as the new jumping and crouching buttons are A and Z which can be somewhat wierd. Duke Nukem 3D is one of the earliest FPS's to allow for the modern WASD setup so if you configure the controls right, you can play Duke Nukem 3D using the same setup that all of the modern games use, which is a big plus. I highly recommend setting the game up for a WASD setup as it makes the game alot more fun to play and more comfortable.
A lot of Duke Nukem's one liners are pretty epic and sometimes funny. When you get to certain areas, he'll usually make a quote and sometimes he even quotes other movies such as Army of Darkness. The sound effects are also pretty creative and they did a good job in the sound department especially with the sound engine coded in which allows for a lot of sounds to play at once and with echo effects being used in appropriate spots.
Although the music is MIDI, this is without a doubt the best MIDI soundtrack to ever grace any DOS game. All the music is well done and is composed to be awesome. If your playing on real hardware, I recommend getting either a Gravis Ultrasound, AWE32/64, or better yet, a SC-88/SC-55 to play the music as the OPL2 Soundtrack doesn't do the game justice.
Duke Nukem 3D runs off the Build Engine which was coded by Ken Silverman and although it was blown away by Quake a few months later, the game had some of the best graphics ever for a 2.5D game, the textures are realistic and the sprites for the aliens and weapons are very well made. The level designs are also very excellent in the graphics department with every texture being perfect and the sprites being aligned perfectly in every way possiable. Although the game runs at 320x200 by default, you can run it at 320x400, 640x480, and 800x600 which allows for greater detail.
Duke Nukem 3D was and still is one of the best 90s shooters to ever come out. It had everything that gamers wanted and so much more. Granted, some people might be offended by the violent and sexual content in Duke Nukem 3D but if you don't mind all of that, then it's a game that your gonna enjoy for a very long time.