Deliberately played vanilla (with the exception of Eternal Doom HUD and Stomper weapon sprites) to prove that it's not necessary to dress everything up ridiculously and use a bunch of other people's mods to make your map fun. You might enjoy playing the map that way and that's great, but the rest of us might have our own preferences. (In particular, big-ass music WADs are frowned upon, mostly because music taste is VERY subjective. I, for one, can barely stand any of the default Doom/Doom2 tracks anymore; I have to play with prjdoom.wad, which is still MIDI but makes every song not suck.)
Stealth monsters are not particularly fun to fight in this amount. Use them sparingly in a WAD; IMO they should only be used in darker areas where you might otherwise find a Spectre, and even then they should be kept to a minimum.
This guy is stuck in the torch. Move him out a little bit so he can actually wake up and attack the player.
I would hold off on the cacos until Map02, just because they kind of come out of nowhere.
I don't like how the shotgun guys just "appear," especially when you just hit the first switch. If you're insistent on them teleporting, you should make sure they actually visibly teleport in and make the teleport noise; AND it should be at the same time the door opens, not once you press just one of the switches. Thinking back now, I'll bet every time you tested it, you pressed the left switch first, right? I always pressed the right switch. Account for this kind of thing.
If I were doing this level, I would just put the Shotgun guys behind the door, and let them attack the player when it opens.
Not immediately fond of the Quake-like "multiple switches for one door" thing, but I never was, so if you're going to do that, all I can say is to keep it more to a minimum. The entire level is a weird switchhunt into little corridors and I'm not always immediately aware of what I'm doing or where I'm supposed to be going. This is a beginner problem; some of my early maps had this issue too. If you're going to do Important Plot Doors with Switches, they should at least spit out a "This Door Is Opened Elsewhere" message. Like Quake did.
I would honestly recommend working within vanilla limitations first, before starting on ACS, but that's me.
I would dress up the main door to the exit a little more, make it feel important, maybe make a long hallway leading down to it. Also, like above, I would change the way the "cutscene" and such works; it's better to just go the Doom 2 route and make the player exit the level as soon as they pass over the linedef. Players are used to that; make them feel comfortable with the familiar. (Doom players love nothing if not familiar.)
Not a fan of the way you used COMPBLUE to border a blue key door, that always throws me off when people do that, especially because COMPBLUE doesn't always look right from a distance with certain colour maps.
Other than that, I can't complain much, and most of these are nitpicky. It's a great first map for a megawad and it has a lot of potential; I just hope the rest of it gets better and avoids annoying problems. (The stealth monsters really are an issue.)
IMGUR ALBUM: 
