I (finally!) have quite a lot of things to say about the map. I know that you've already submitted it, but I'm hoping I'll be able to persuade you to change a few things.
The map almost looks like it was put together by two people, due to the difference in quality of the best parts of the map and the worst parts. I think if you can smooth out the rough edges it'll be an impressive map.
My long list of comments is as follows...
I think a physical barrier should be added to the cliffside here. At the moment you have some impassable lines preventing you from going down into the bit outside the gate, but it looks as if you should be able to get down there. Also, the area outside the gate is completely empty and seems a little bland. Add a building going into the wall with the suggestion that you came from that building before entering the installation? Adding little things that the player can see but not access makes the map seem more like an installation and less like a small bunch of rooms hacked together by a level designer. Good examples of this are maps like Alien Vendetta 02 and Kama Sutra 01.
Also, the secret up there is way too easy to find. Not really worthy of the name.
This is an issue with all of the outdoor areas in the level. The rock faces look artificial. This is probably because you haven't used enough lines, and it looks too jagged, not curved enough. Especially noticeable where the rock meets the sky and where the rock meets the building.
I think this area would look better if you'd make the sky higher so you could add higher cliffs that would make the area look more believable. At the moment the cliffs just cut off once they reach the ceiling in a way that's not entirely believable to me. I probably didn't make that very clear, but I'm hoping that if you look at the borders to the outdoor starting area of Capellan's map, you'll see what I mean.
The texture with the red UAC symbol looks odd here because of how the brown lines on it suddenly get cut off.
I think the upper texture in the sky sectors doesn't contrast well with the ceiling of the room. Also the walls in this area seem too bare. You could add wall lights, different light levels would make things far more interesting.
The contrast between the roof and the upper texture seems even worse here. The wall texture is too repetitive given the specific details it contains, and not enough lines have been used at the corner (not smooth enough). Walls too bland.
What really jumps out at the player here is the weird lighting effects of the slime, in particular the patterns on the ceiling. Apart from that, the ceiling is too uniform, and the SHAWN texture seems out of place here, as does the button texture. Again the repetitive wall texture. The roof gets cut off in the second screenshot. The third contains a texture misalignment.
This room suffers from most of the problems of the previous room, mainly the slime lighting and the SHAWN crushers, but also the emptiness of the walls and ceilings. Worst of all there seems to be no way to get out of the slime if you fall in, which is very annoying. The plinth holding the key is nice though.
Again the upper texture. Apart from that this area looks great.
I'd prefer some less jarring way of displaying that secret area. The transition between the cave and the secret room doesn't work well either, as you can see from the second screenshot. Try making the floor of the secret room lower and making the floor of the doorway the same as the floor of the cave, perhaps? I also prefer it when secret areas have something in common with the surrounding area. This sudden little room of hell looks ill suited to the surroundings.
This room looks smoother than I'd expect in a cave, add some sectors of varying heights perhaps? The door leading in is far too thick, n my view.
Crushers look strange to my eyes. The crate looks even stranger. Try making the part that has fallen onto the floor separate from the rest. That door isn't going to work, and I think the cavern would be better off without it. I really like the glowing floors you've used to border the slime river, but the green textures on the rock face don't fit at all.
This hall seems a little too unadorned.
If you're not going to actually make an outdoor area for you to see when you look out the windows you should probably not bother with windows at all. What's currently there looks bad.
The room at the top looks odd for reasons I'm not entirely sure of. Probably the sharp corners and the fact that the doorway is extremely deep set. Use of lighting could again make things a lot less bland.
I hate the floor texture here. Almost anything has to be better than this. And if that area you're looking down on is meant to be the start area, then shouldn't you be able to see the place you're currently standing when you're back at the start of the level? I suggest you make it clear that this is a completely different area. You can play games with perspective like that in hell, but not in an installation :P. Again there are impassible lines without visible barriers. The other problems here should be apparent from the last two screenshots. The walls of this area also look pretty bare.
I don't like this secret room. First of all it's extremely difficult to see (only a tiny texture misalignment on a wall you're probably going to pass without a second look). Secondly, it has nothing in common with the rest of the level and looks weird (especially the blue platform's texturing). Thirdly, the items and monsters in there seem like overkill. The area outside the secret room is shaped oddly and the colour of the door contrasts badly with the colour of the walls.
I didn't like the look of this room either, for some reason. I suspect it's primarily to do with the metal bars texture which I think looks too flat. The walls and ceilings of the room again look too bare.
These rooms could do with some sprucing up. The second especially.
I really like the fleshy bits of floor here, but apart from that the hallway is too bare. The ceiling and walls are crying out for more detail.
Again the floor texture here is bad. I think the green force field walls look rather out of place as well. I'd prefer walls of stone to match the buildings the mancs and cyb are standing in.
Again the borders are too jagged, and the lighter brown texture looks out of place.
The lava pool is nice, but the wall texture above it looks odd. I seem to recall the flat's animated but the texture isn't? Or maybe it's just that there's no border between the wall textures and they clash.
More screenshots where the walls weren't smooth enough.
The gameplay could also use some retouching, I think. Archviles were massively overused. There were about six of them, and some of them were used pretty unfairly in my view (the two teleporting ones). There was also a cyberdemon and a mastermind. The monster count was pretty low, due to the high number of heavyweight monsters used, but most of them weren't very difficult because of the weaponry you were given. I'd say you should increase the number of lightweight monsters such as imps, add more medium monsters like hellknights and cacs and reduce the numbers of viles, cybs, etc. I also think the BFG is overkill.