More flawed than you remember.
Doom 2 isn't a bad game by any means. The fact that it's still fun to play more than 10 years after it was first released is testament to that. What you probably don't remember is that Doom 2 also had a lot of painfully bad levels.
Hell is invading Earth, and it's up to Doomguy to save the day. Using the original stable of weapons, along with a new toy, the super shotgun, you'll do exactly what you'd expect from a sequel to Doom. You shoot a lot of demons, collect keys, and shoot some more demons. Early on the game is pretty good about easing the player into things, generously placing health pick ups and making the key hunting fairly straightforward. Then it kicks you in the nuts and the difficulty curve skyrockets.
Remember how even the most complicated levels in Doom had some sort of logic in their layout? The same is hardly true for Doom 2. Most of the levels are oversized mazes, and the ones that don't look like someone took an empty space and just started placing things on the map in a drug induced frenzy. Thankfully there are some fun and enjoyable maps in the mix, but it's pretty depressing when the part of the difficulty comes from not knowing where the hell you're supposed to go. You'd expect to see this kind of map design in a fan WAD, not the actual game.
In addition to the obnoxious maps, a few new enemies are on hand to piss you off. Pain Elementals resemble brown Cacodemons, and their main form of attack is spawning more enemies. Lost Souls were usually just a mild annoyance in the first game, but now that they can suddenly fly at you from the Pain Elemental means they hit more frequently, and in much larger numbers. And the best part is that when you kill the Pain Elemental, more lost souls spawn on it's corpse. So if you got trapped in a small room with a quickly growing number of Lost Souls and a Pain Elemental, your reward for surviving the beatdown is more enemies. They also spawn in larger numbers based on how much damage the Pain Elemental took over time. So that's nice.
The only other new addition to the rogues gallery worth mentioning is the Archvile. They can set you on fire as long as they have a clear line of sight to you, and that fire deals enough damage to drop you from max health and armor to a mere 60 health if you give them enough time to light you up repeatedly. Unfortunately, that's not the worst part about them. They can also revive dead enemies. There is nothing more infuriating than mowing down dozens of enemies, only for an Archvile to come out of the wall and revive all of them. So naturally you'll want to kill them quickly, which isn't easy because they have enough health to survive about 4 or 5 rockets before finally eating it.
Thankfully, the other new foes are all pretty fun to fight, and when something like a Mancubus is grouped with some imps, the hail of fire hurtling towards you is potentially devastating. Even the Pain Elemental compliments the returning roster in interesting, if not sadistic, ways.
Despite the poorly made levels and enemies that are a chore to fight, Doom 2 manages to still be fun as hell. Something about the fast paced madness of it all is exhilarating, and every enemy you murder is as satisfying as the last. Completing the lackluster levels energizes you, knowing you just overcame large amounts of bullshit because you're just that awesome. Dropping multiple imps like flies with a single shot from your super shotgun is one of the most enjoyable things in gaming, and the inclusion of two levels from Wolfenstein 3D as secrets is pretty neat. Everything that made Doom fun is still here, and you'll still have a good time. You just need to be willing to deal with the low points.