Howdy! I'm a mapper who likes to write reviews of the campaigns I play, but since you've asked for feedback on a campaign in progress, I'll be addressing my concerns directly to you, the author(s). I couldn't fit my thoughts into a comment, so I hope this discussion post will serve you well.

As of May 31, 2021, Dr. Malroy's Laboratory is a campaign full of interesting ideas that still needs a lot of polish. I appreciate the creative atmosphere and the fact that the campaign isn't too difficult or too long, but I would say it has a way to go before it feels finished.

Please feel free to disregard any of my suggestions if you don't like them -- it's your campaign. Here's my feedback in bullet points:

• This campaign has a fun, spooky atmosphere. The thick fog and bizarre architecture in particular sells the tone, and I wish there was more of that stuff. It all feels like something out of an old sci-fi horror B-movie, which is perfect for something as popcorn-munchingly cheesy as Left 4 Dead.
  
• As I said above, the length is just right -- surprisingly brisk for 6 maps -- and the difficulty is still perfectly manageable even though hordes spawn at a higher rate than vanilla.
  
• I greatly respect the decision to make a Hard Rain-style campaign where the second half of the levels are just the first half in reverse. You all pulled this off surprisingly well considering the short development time. The arrows were very helpful!

• The fog is almost always too saturated in color, which makes it stand out in a cartoonish way. Maybe this is intentional, but it's especially distracting in the finale because the fog doesn't blend into the skybox color like it does in official campaigns.
  
• Important objects for progression are usually hard to find. I don't mind a little exploration, but finding a tiny hand radio lying on the ground added an extra 5 minutes of wandering to the finale chapter for me. I strongly recommend shining bright lights on important objects and/or the path forward. Valve does it so players spend less time being lost and confused, and it works.
  
• The level design is very boxy and flat. I understand from the Workshop page that you built this in 1 month, and it's incredible for what it is -- you certainly work a lot faster than me! -- but this does leave the levels feeling rushed. All 6 maps lead players through a repetitive series of rectangular rooms and corridors punctuated by similar-feeling crescendo events, and this lack of memorable moments or "peaks and valleys" hurts the pacing. The Whiteboard Test[80.lv] might be a useful read.
  
• This campaign feels like it's lacking context. I know story has never been a top priority in Left 4 Dead, but I was never motivated to do anything in this campaign because things seemed to happen for no reason. In No Mercy, for example, you have a clear goal from the very beginning: get to Mercy Hospital. In this campaign, I never had a good idea of what I was doing or why I should do it. Adding Survivor dialogue and more crescendo hints could help with this a lot.

• My favorite part of the campaign was when the Survivors had to avoid the electrified water in the floor -- it's extremely memorable! However, I think the execution is overly punishing, especially since Tanks can spawn during this section. Instant kill obstacles should be used sparingly in L4D2 because restarting a chapter after a death is extremely demotivating in single player. I think the water obstacle would be more interesting if the electricity only zapped you once every 5 seconds or so, and each zap only did 20 damage. Add some warning sound effects and a screen shake when the zap happens and I think it could be an incredible moment.
  
• I was missing textures in the underground labs. I think they might have been signs above the doors, but I can't tell you for sure because they were missing (obviously).
  
• I'd like to see more secondary weapon spawns, especially in the later maps. There aren't many opportunities to swap your pistols for melees and vice versa.
  
• Why does the garage door in the finale "open" by touching it so hard it breaks? That was so unintuitive I wondered if it was a bug.
  
• This could be a bug, but a lot of the crescendo events didn't have a "Be ready to fight the horde!" warning flashing over their associated buttons. I think it's important to have these warnings present so players aren't surprised with a horde they're not ready to fight.

Overall, I'm really looking forward to where this campaign goes from here. It has a lot of potential, and y'all should be proud of what you've accomplished so far in such a short time.