If you thought DOOM was great, wait until you get a load of this. More levels and the alimighty Super Shotgun. When a shotgun doesn't cut it, get in the demons face and delete them with the super shotgun. Seriously, this gun practically trivialized any lesser demon, especially since it could already take 2-3 shots to kill some of them depending on your aim.
DOOM II also added new enemies, giving the designers and modders more tools for variety in map making. The game tech wasn't much different from the first, except for larger maps and support for bigger enemy crowds. While not big changes, these set DOOM II as a basis for the majority of DOOM mods to come after it. The non-episodic level progression in this game versus the first also meant a player could retain their inventory through out the entire campaign, instead of resetting it in each inventory.
Not everything is exactly better here, and there's a reason why it isn't higher despite the improvements. The biggest reasons are the level design and the use of new enemies in this design. The original game presented a story of sorts of gradually going deeper into a Martian complex before you're sent into Hell and eventually more grotesque landscapes before dueling with the Cyberdemon. Doom 2's level design feels less gradual and more random. Further more, there are more level design trappings that fans started to not love, such as platforming while dealing with long range enemies, extremely narrow pathways, and awful traps that leave you surrounded by chaingunners with long range, hit scan attacks or a cheap (but admittedly cool) hallway filled with explosive barrels that can easily kill you if you're not careful. Add to that, the awful archvile that not only creates pillars of flames that are extremely damaging and perfectly accurate unless you hide behind a wall, but also resurrects nearby dead demons - so while you're recovering from their attacks, they are making their resistance even stronger.
Truthfully, these changes did add some needed variety to the game. I feel if DOOM 2 just did what DOOM I did, but just with more levels and a few more monsters, then it wouldn't have been as special or necessary. Like I said before, DOOM II is a basis for many mods and level packs, making DOOM II in theory, a superior game to DOOM II simply for being the same game but with improvements and additions, and this as a whole package is why DOOM iI is so high on my list, even if the game on its own is more irritating to play at times than DOOM II in my opinion (but also more fun in other situations). That said, the Chaingunner traps are unfair, and I like the challenge of a hard enemy like the Archvile, i feel he could be redone a bit to not feel like such a roadblock to the flow of game.