HLM2 came close to real greatness, and then it just smashed into a brick wall.
The complaints I have are few, but in my opinion break the game. The switch from almost claustrophic level design to wide open spaces, as well as the sheer amount of enemies with guns stationed at windows, actively discourages from the best part of the first game, speed. The greatest moments in HLM1 were when you cleared an entire level without stopping in one fluid, almost graceful scene of gore, and you replayed the game to try and get faster and get longer combos. You cant do that in HLM2, instead you have to be extremely cautious and play as a sniper, or abuse the AI and bait them.
These wide open spaces also lead to a new importance on guns, the least interesting part of Hotline Miami's combat. Melee was the bread and butter, and guns were relatively rare because they attracted too much attention. Now you NEED to use guns and its just not as visceral as it was.
My third complaint, the masks are gone, and in their place we have a dozen characters, most of whom have few game changing gimmicks and tricks. The Fans (and a couple of others I wont say in case its spoilers) deliver the most variety, but the other characters are just dull. Ironically, HLM1 had far more variety in its gameplay and it's a fraction of the length of its sequel.
I'm going to not spoil the story, but theres a moment around halfway through the game where it just all comes tumbling down (you'll know it when you see it), and it was after that the game became more of a hassle than an enjoyment.
I loved HLM1, I played through it a week ago to get those last few As I needed, and HLM2 is just a pale shadow in comparison. I'm sure a lot of people will like it, but its just disappointing to me and a lot of people I know who were looking forward to the game.
Soundtrack isnt bad though.
EDIT: Okay the soundtrack was actually great, and Rust is the best god damn song on it.
The best thing about HLM2, my favorite thing, was the atmosphere, that feeling of death, madness and impending doom that saturated the game. You could hear it in the music, see it in the growing destruction and decay of the city in the loading screens, and it was disturbing, in a good way.