Recently I've played a bit of Alien Swarm, and I think that once you've played one level, you've played them all. That doesn't mean it's boring, oh no. I still find it frightfully fun. Between the four classes each seems to have its feature that sets it apart, making them fun to play for different moods. Plus, the fact that it's multiplayer means you're often left pushing yourself harder to make up for that one guy who just couldn't figure out that fire is hot and spikes are sharp.
My concern is that all the maps look the same. It appears as though they've tried to make them unique, and each level has its own interactive gimmick, but the textures just look the same when I rush by them, missing all the detail and hard work they put into them. This is not unlike another game I've been playing on and off since it was released, Diablo II.
Yes, Diablo II's maximum graphic resolution is 800x600, yet I don't care, for the exact reason I don't care about Alien Swarm's graphics; I don't see them for long enough for it to matter. I run from here to there, the monsters become corpses, and the only time I care to look at the low-res images of my inventory is in decided what to equip or vendor, and then I'm more concerned about the tooltip text.
So why would I ever look at graphics? Only to figure out if something is interactive. If I can talk to a person, I'm going to want to know what kind of person that is (vendor, quest giver, corpse, etc). If I can shoot an object, I'm going to want to fathom a guess at what will happen. (Will it explode, release poison gas, set free a swarm of gnats, free a trapped teammate, drop some loot?) If it's the ground I'm walking on, I will want to know if its going to hurt me, heal me, or just remain stably under my feet (or collapse!)
I also don't see why every character needs seventeen different attack animations, and an equal number of casting animations. Every single one is as ridiculous as any others, so one would work. Most of the time when I'm casting anyways, I'm not looking at my own character, I'm looking at what it is I'm attacking, or glancing around the screen while preparing my next move. Hardly ever do I care what color my hands are glowing or whether I shove my right hand or left hand forward. (I also don't care if I shout 'Hyah!' or 'Nuh!' at the end of my cast bar.)
The obvious disclaimer applies that if your game exists solely to be beautiful, or a large part of the game is spent on close-up examination of game-world objects, you should definitely go all out.