I had a look at your map - played it through - it was pretty fun!! I played on skill 1. I would throw in the following constructive criticism:
Too much ammo! I dont think you needed to put half as much shells in the map. Also for a map with 26 monsters I dont think you need a rocket launcher or a supernailgun. You would get away just fine in this map with a supershotgun, nailgun and a grenade launcher!
Leaking map - I take it that you tried to run vis for this map, and it wouldnt work. What has happened is there is a leak from the inside of the map leading out to the 'void'. Your solution is to make a massive box around the map and hollow it! This allows vis to run, right? I know because that is exactly what I did for all of the maps (except the start map) for my first release 'DejaVu'. Take a look and see!
It is bad practise. You end up with a load of unused space between the outside of the map, and the inside of the surrounding box. This could be described as 'unnecessary lightmaps', or so they tell me ;-P
There is infact a solution though! I dont know if you noticed but when you have run your QBSP program to build the map it will have, at some point, said something like:
"leak found at (co-ordinates) writing to r_666.pts"
This 'pts' file is called a pointfile. Load your map as normal, bring down the console and type 'pointfile' (make sure your pts file is in id1/maps). This will draw a dotted line which will show you where the leak is. If you cant at first see the dotted line, type "gl_clear 1" and 'noclip' outside of the map, and you should see it! Then all you have to do is go back into your editor and adjust the brushes so that the leak has gone. Repeat this process until QBSP finds no leaks.
When you run QBSP there will be a line which says "outleafs 0" or "outleafs whatever" (in the case of the boxed map). This should be a positive number.
For more info, look to the documents at the bottom of this page here, AguirRe's site:
http://user.tninet.se/~xir870k
Neg|ke is right about the sky platforms - they dont quite look right.
Apart from that, for a first map its actually pretty good! Some of your brushwork is very impressive, lots of curves and angles, reasonable texturing - impressed by the lighting :D Just a few technical bugs to iron out and you'll be rocking!
I enjoyed playing it, thanks!