Everything a sequel should be. Doom 2 is about as close to perfect as it gets.
We've all heard the saying, "if it ain't broke, don't fix it." This is definitely the truth with Doom 2. ID has made virtually no changes to the core gameplay which is a great thing. You still play as lone space marine armed with enough firepower to level an entire city. Your only goal in each level remains to find the exit to the next level and kill any demons you encounter. No there's still pretty much no story but really, does Doom need a story? I sure don't think so. All you need to know is that there's a bunch of demons running loose and it's your job to annihilate them all.
Probably the biggest difference in Doom 2 is the difficulty. It's much harder than the original Doom. I guess ID figures you've probably already finished Doom a few times over so this time, they're not holding back. The levels are much bigger and more challenging and they're stuffed full of huge numbers of demons. You'll be encountering large groups of monsters around pretty much every corner and the new enemies, which I'll be covering next, just make it all the more difficult and at times, frustrating. But hey, no one ever said saving the world from endless hordes of monsters would be easy.
As I mentioned before, Doom 2 features several new enemies:
Chaingunner: You thought Shotgun Sergeants were bad, wait till you get a load of this guy. He's much tougher and carries a chaingun that he fires relentlessly until either you or him are dead.
Hellknight: Basically a scaled down version of the Baron Of Hell. He has only half as much health but his attacks are just as strong as the Barons.
Revenant: A walking skeleton in a suit of armor who moves quick and fires homing rockets.
Mancubus: A huge behemoth who packs a pair of flame launchers that cause some serious damage.
Arachnotron: A miniature version of the Spiderdemon who has a plasma rifle instead of a super chaingun.
Pain Elemental: The most annoying enemy in history. He spews out an endless number of lost souls at you and even splits into a pair of them when he dies.
Arch-Vile: Without a doubt the most diabolical creature ever conceived. Not only does he cast a extremely damaging fire spell against you, but he can also revive nearly any demon you kill! Believe me you're gonna learn to hate this guy real fast.
Sound effects haven't changed at all from the original Doom. You still hear the sounds of the Demons moaning and growling all around you. Your weapons have all the sounds we've come to know and love. The only difference is the music. As the GS review pointed out, it's definitely inferior to Doom's soundtrack but it's does the job well enough. In any case you don't really play Doom to listen to the music anyway.
Doom 2 is doesn't stray too far from Doom's footsteps, but it definitely improves upon nearly everything that made Doom great and thanks to it's huge fanbase who are constantly pumping out new maps and mods for the game, it doesn't look like Doom or Doom 2 will be dying out any time soon. Doom 2 receives my highest recommendation!