DOOM II was built on the considerable strengths of its predecessor to provide an unforgettable action game experience.
DOOM II trades in the lunar labs of the early DOOM episodes for a more familiar locale: Planet Earth. True to its subtitle, Hell's minions have figured out how to invade Earth, slaughtering untold billions and perverting the places they roam into warped versions of their former selves. Humanity's remnants plan to escape the ruined world in a massive spaceship, but the spaceport is currently occupied by hellspawn. As the sole survivor of the original demon war, it's left to you assault the occupied spaceport and secure humanity's future.
So basically you shoot stuff and try not to die. It's DOOM, ya knucklehead.
Harder, Faster, Better, Stronger
The most important addition to the game is the double-barreled shotgun, which is simply one of the best weapons ever concocted for a video game. It's over twice as powerful as the original shotgun but has a much less focused spread, ensuring the regular shotty isn't obsolete. It's powerful enough to take down even medium-strong enemies with only a few well-aimed shots, which gives a nice boost to the pace of the gameplay and encourages more close-range combat. It fires with a deep, bass-heavy "CLICK-CLACK-BOOM!" that never fails to be satisfying. Best of all, it can take down other players with a single well-placed blast, forever altering the dynamics of DOOM deathmatch -- it's now even faster.
You'll need this extra advantage against the new monsters, too. Each one is a perfect addition, and I always miss them when I go back to the original DOOM. In DOOM II, id fleshed out the ranks of medium and upper-tier enemies. The walking skeletons called Revenants have strong offense but weak defense, making them fun but scary cannon fodder. The disgusting rolls of fat called Mancubis (Mancubi?) fill the battlefield with chaotic salvos of three-way fireballs. Pain Elementals live up to their names, dispensing unending streams of irritating Lost Souls. Arachnotrons are like little Spider Masterminds, but with fast plasma guns. And the Arch-Vile is simply the most evil enemy id ever created. Not only can it resurrect dead comrades, but it has a nasty line-of-sight attack that can nail you for over 80 health a pop. If you don't hide fast, you're toast. All told, these new enemies increase the difficulty and diversity by an enjoyable degree and give your new shotgun plenty of workout.
The new levels, too, are an improvement on the old. While I'll always have a place in my heart for the classic DOOM maps (episode 1 especially), it's clear that id was much more experienced in level design by the time it made DOOM II. Outside of a few gimmick maps, the layouts tend to be more interesting and diabolical; some traps are downright nasty, and the designers make excellent use of the new enemies and their unique abilities. (Map 13, "Downtown," is an all-time great.) Even the textures tend to be well-aligned, which was a sore point in the first game.
Overall, DOOM II is somewhat more dingy in appearance, with lots of browns and grays. This lends itself to an ultra-depressing tone, augmented by almost unbelievably grim music. Some may prefer DOOM's more cartoonish and energetic tech aesthetic and high-speed metal, but I actually like DOOM II's darker, more moody nature quite a bit.
Heaven on Earth
DOOM II is an excellent sequel, subtly building on the merits of the original game to create an even better, more complete experience. I suspect that those who were disappointed by its lack of sweeping changes were probably not too big on the original DOOM in the first place. DOOM II polishes the DOOM experience to an ultra-bright sheen, and still retains its status as the most complete-feeling package that id has ever created. When it comes to out-of-control, edge-of-your-seat FPS action, DOOM II is the real deal.