A fairly good map set you've created here, though with some definite need of refinement.

For map design, asthetics wise, the opening and first map were rather well done, although the second map was very up and down with some areas feeling amatuerish and dreadfully unpolished. Overall, above average in purely visual design, for the first map, below for the second. Things clearly need a couple more passes, art wise.

The maps didn't feel like actual places however, save for the initial "outdoor" area. Everything felt artificial and "there because game".

For map design, gameplay wise, you've got some issues:

Poor signposting in areas is a problem, I suggest the use of the vox announcement system to help guide players around. Pressing a button and hearing a "blip" doesn't really tell you much, other than something happened, but pressing it and hearing "Secure Door Opened" works much better.

Teleporter puzzle was decent, could have used better signposting that the valve was in the prev room. Same with the grunt. Steam or sparks work well for drawing the players attention.

Encounter design... is very poor throughout, honestly. Either trivial or artificially difficult were the only two difficutlies present in them, save for one or two. Once again, first map was signifigantly better in encounter design, save for the first marine fight.

That vortigaunt encounter in the apc tunnel in the second map was just, cmon man, more mans is not more betters. You coulda cut down on the number of gaunts and added a couple houndeyes or a grunt for an actual interesting encounter, instead of just... more mans. It was stupid and felt incredibly amatuer.

Same deal with the catwalks earlier in the second map. Honestly, I started expecting to see 50x zombies in a grid-pattern in a tunnel at some point.

Blindsiding turrets are also not fun, especially if you get no warning of them. Use some tripwires, or pre-activation so you know what's coming and can react. And that turret wall at one point was just... plain dumb and silly? and was unavoidable damage, as if you got close enough to identify it, your going to take at least 30 damage. Always, always avoid unavoidable damage, just as a general thing. The only exception is if you have unavoidable healing beforehand; you never really know how much health a player will have. (Though a sign of good design is having a rough idea of how much they'll have at any point)

What was the point of the excessive amount of tripmines? The player has grenades [/i](And clear LoS with a gun from a safe-distance)[/i]

First combat encounter with marines was iffy, no time to get your bearings in an area without immediatly identifiable cover, which then quickly turns into conga-line murder once you go inside the building.

Nitpicks:

Those powerups in the dead end direction of the vent. Don't waste a players time. That could be signifigantly shorter crawl distance for those pickups.

Long ass vent crawl at one point. Not actually super long, but long and unaccented enough to be irritating.

Barnacle placement in first map, bottom of overpass by ladder going up, they're all in a perfectly straight line, looks artificial.

Not being able to turn the valve for the methane tank in the tele puzzle. Felt weird, and made the puzzle more opaque. (Did not realize a valve was missing, thought the tanks were just set-dressing. If that valve had turned, would have imediatly gone "things need doing")

Bugs:

Going back to the first map from the second results in the player getting placed out of bounds.



In conclusion, you've got some good potential here if you want to continue to refine the set. Will definitly be looking out for any future maps you make, you are a good mapper. But you need to work on your encounter design. And put more thought into the flow of your maps. And optionally put some focus into making a map feel like a real place, least if you want to go for that school of design.