Before I start, let me state this: Doom 2 is my favourite IWAD, and I love the levels and gameplay in it, so I don't HATE it, just pointing things that I thought could have been better.
ALSO: I know of limitations during that time period, so what I say might not fit up to old PC standards.
One thing I wasn't very fond of was some level design. I didn't like some of the maps because some were kind of weird and unfitting, like Barrels O' Fun. It would better suited as a secret map, rather than an actual level. With some levels, I wish there was more to it, like The Chasm. You're usually just walking on lines, then falling off. It's one of those levels where you wish the engine was fully 3D, bringing a whole new challenge. (Those people only waited 2 years from then...)
This brings me on to the next point: not a lot of engine differences. From what I know, all that changed were a few more linedef actions, new AI for the new monsters, a few new items, and one extra weapon. I really wish they expanded on the engine more, rather than what they already did, adding a few new things wouldn't push a 1994 computer to the limit, would it? But then again, I actually don't know what else they could've added, besides things you would've found in Heretic, so yeah. Take note that I LOVE the Doom 2 monsters, so don't think I hate them. I find that they're a new twist to the Doom monsters.
Now, probably the biggest disappointment to many, the lack of new weapons. I have open arms to the "Super Shotgun", but I wish that even if they made one weapon, I wish it would've been something new, instead of another shotgun. Not to mention, due to the hard-coding, I'm not sure if people could add or take away weapons to vanilla Doom. (Didn't know Doom in the 90's...) Not to mention, it was pretty overpowered, and you find it on the second level! The Doom Wiki says that it's as strong as the rocket launcher, which I may add, you find non-secretly on the third level. Gosh!
Finally, the final boss. I had more trouble trying to shoot my rocket launcher at the right time than I did avoiding those damn cacodemons. It could've been better, but at least it wan't another Spiderdemon, and I thought that the final boss looked cool. Non-sensical, but cool.
That's it. The game is still good for what it is. And remember: this is my opinion, not a fact, but I'm pretty sure no one gives two shits about my opinion anyway, so GOOD DAY PEOPLE!