Although I wouldn't go as far as to say that Doom2 was terrible, I do basically agree with the sentiments of the OP. Much of what I loved in Doom, I was underwhelmed by in Doom2. The maps were, for the most part, nothing particularly special IMO and nothing like as cohesive or immersive as the ones in Doom were for me. I didn't particularly like their logic, layout architecture and their lack of sense of place. Even the art direction of many of the textures was a bit dull and meh as far as I was concerned. And the music, oh yes, do I agree with the point about the music! The original Doom has one of the best game sountracks ever IMO. I was blown away by it in 93 and I still love it. The Doom2 soundtrack never gave me that same vibe. There are very few stand out tracks and those that are, are nowhere near as stand out as those in Doom. Much of the music in Doom2 felt, and still feels, quite inappropriate to me.
And the final boss is a wall with a hole in it. A rocket post box.
I think I slightly prefer the episodic structure of Doom over the one-game structure of Doom2 as well.
But then there is the extended bestiary. That was a significant addition to the mix. Some of the Doom2 enemies are excellent (and some are bloody annoying). The SSG, I can actually take or leave. I love the original shotgun. I think they got it just right. I don't think the SSG was really needed and it can actually be a bit unbalancing. I don't mind it being there but it's not that important to me.
So, as far as I am concerned, Doom beats Doom2 on almost every front except the bestiary. I remember being quite underwhelmed on my first play through of Doom2 and that hasn't really gone away. Of course I played it right through but I haven't done it that often since. I just don't like it enough. However, by the time Doom2 came out, I was already spending much of my time editing Doom and so Doom2 served primarily as an editing resource for me. I suppose it was kind of like an official Realm667-like resource to me long before the concept of such a resource was born.
i cant believe im about to type this.
youre right. its true. youre right.
im not being sarcastic.
Agreed, the new monsters definitey did add, the mancubus were fun.
Pain elementals feels useless in the game, all you need to do is run for him to get a few kisses. Seriously, these guys are helpless most of the time I chase them, they need a melee attack to prevent abuse of such bug.
Revenants and mancubi are probably more your allies than your enemies, you can literally use revenant's rockets to your favor, mancubi are constantly infighting the other monsters, besides, they are pretty helpless alone.
Skipping hell knights...
Chaingunners, well, you can make infights with them, but you get fucked easily by them, my bad, I will step back and say this zombie is awesome, a rape machine, they deserve respect.
Arch-vile: FTW, nuffin' said.
Arachnotrons feels like a copy of the mastermind, i have yet to see them being used effectively like in Dead Simple, they encourage infighting a lot, bleh.
Wolfenstein SS feels redundant to the chaingunner.
There you go, these are the reasons i don't like many of the newer monsters, most of them are because they infight more frequently, discuss.
EDIT: SSG - This weapon has a surprisingly high damage output (higher than RL!), so high that you will often get mad at a spawn fragger, there are no penalty for the user who loves to smack your face and then killing you with a single SSG blast once you respawn, no time to switch weapons.
Yup, the Pain Elemental and Hell Knight were GARBAGE.
Pain elemental is nothing but a baby-faced cacodemon that spawns the most annoying enemy in the game and doesn't even make any sense.
Hell knight is a baron with less hp, less damage, and shaded a different color. This reminds me of some spawns in WoW...
As for the SSG, there's no denying it's fun. Maybe its a bit OP. I like the original shotgun which is an INCREDIBLY diverse and useful weapon. But ssg and a couple monsters don't make up for doom 2 being the sorry excuse for a sequel to doom 1 that it was.