I've played through most of this thing, am now up to EP4 with 3.5 levels of it under my belt. Decided it's time for some feedback. Note that I've never tried modding, but have played quite a few quality megawads until now, both for vanilla and ZDoom.
General mapping tips:
-try to make each map unique on it's own. A player is generally interested in seeing different layouts and interesting ideas, not seeing the same level three or more times in a row! (note:this generally applies to most levels in EP1 and EP2);
-don't use the same gimmicks in every map: A differently colored wall texture can make for good secrets in the early levels, but try to make secrets in the latter levels more interesting/harder to get than that. It's boring to see the same secret throughout a wad, especially one with 40 levels;
-try to use automap secrets for weak secrets (invisibility powerup, a bit of ammo), not for big ones like megaspheres (see:map08's megasphere secret);
-don't use copy-pasted rooms unless you can use them in a creative way (ex:after walking in a copy-pasted room just like the one before and killing the few imps there, you press a switch, only this time, instead of opening a door, it makes the room go pitch black and teleports in 3 archviles);
-avoid making a level feel tedious/boring (the Tityos Asteroid level's first half feels way too bland/unimaginative and generally repetitive);
-avoid the use of pseudo-hubs (the first three episodes are designed as hubs, but you can't return to previous maps so this makes no sense);
-if you use a texture for something that isn't a damaging floor, make sure you don't reuse it in the same level for a damaging floor (unless you put "poison" signs);
Custom message tips:
-don't use a large font for custom messages;
-don't make custom messages for painfully obvious things such as doors that you can already see opening while you press the switches/know were supposed to open;
-try to make custom messages unique and original ("you hear a peculiar noise in [area]" sounds much better than "you hear a door open in [area]";
-try to not use the same idea in every message: "you hear some" is repeated in nearly every custom message. I was, however, pleasantly surprised by the one in River Phlegeton where it said "the river now flows deeper in the underworld";
-don't have a repeatable action use a custom message every time:only show the message the first 2-3 times (an annoying example being in the Vault of Flesh map)
General tips for jumping sections:
-try to make your jumping sections of easy to medium difficulty if the WAD isn't mostly based on them;
-make jumping sections more interesting. Utilizing the same type of platform to death will make players hope the section is over fast instead of having them enjoy it. Make use of differently shaped platforms, maybe some moving ones. Be creative!
-don't punish missing jumps too much. Having a 20hp/second lavafloor below will surely make any bad jumper quit and just use a cheat;
-for long sections, make sure you implement some checkpoints.
General monster/ammo/weapon placement tips:
-unless your level set consists of a hub (basically, this applies for the first 3 episodes), try to make sure every level is finishable from pistol start, without finding any secrets;
-if you, for whatever reason, don't want to make levels completable from pistol start, don't taunt the player by giving him weapons he already has/should have from other levels, or by making only some of the levels completable through pistol start (basically, you can finish every level with max kills through pistol start up to map08, which you can't. This makes absolutely no sense!)
-try to avoid placing important items (ex:ammo caches, powerups etc.) at the end of a long/difficult to navigate area unless you are SURE the player will need them right away. Otherwise, the player will have to backtrack a long way to get them (ex:one of the Soulspheres in the
Vault of Flesh level);
-if your level set is a hub that's not full of slaughter-maps (EP4), try to avoid giving the player the BFG/any overly powerful custom weapon in the first half of it (and make absolutely sure not to give it after 2 levels like you did). This will just make the player overpowered and make most fights too easy;
-do not make a map like Vivarium ever again! Not only is it executed poorly, but it's also based on a horrible concept;
-if your wad is for (G)ZDoom, try to consider making use of themed monsters/weapons for your level set to feel more consistent (ex:bacteria enemies for Vault of Flesh, bows, clubs and other primitive weapons for castle and egiptian levels etc.);
General tips related to music and sounds:
-you can always opt for an in-level music change for important sections like boss battles or the invasion on the first spaceship level;
-try to adjust the length of the song to that of the level:if you have a short song for a long level, people will get bored of it;if you have a long song for a short level, people won't hear all of it;
-it is best to choose songs that sound good both in OPL emulation and in MIDI. This way, people can switch between the 2 modes when they get bored of one;
-try to avoid taking sounds that are common out of context:ZDoom's secret sounds usually triggers when you find a secret, and it sounds kind of like discovering a treasure. It doesn't make sense to use it in the Vault of Flesh to indicate that stomachal acid is flooding.
General tips related to the story:
-make sure the premise of the story is plausible (why would a super secret base housing such important research have only a few poorly armed guards? You can opt to include robots (Strife is a good source), turrets and marine with BFG skins to better give the idea of "super secret base"...however, I do realize this IS the UAC, so anything is possible);
-make sure you don't have parts of the story that contradict themselves:upon finding the secret rift, the player is given the option to "explore", even if it's clearly leading in the wrong way. The player is on a mission that he must finish ASAP or the whole humanity might die! He can't just "explore", take a nap, or decide to suicide with the rocket launcher! Better way to implement it:
"Sgt. Riley:Dammit marine, looks like that was a one way teleporter. This chamber is sealed and I can't access any controls to get you out of there. Hurry up, go through that rift, and pray to god there's a way to come back to the ship again!"
-make sure the characters' personalities don't clash with what they are saying/doing:upon entering the mine level, Riley says "looks like this isn't a simple cave, marine. It's a mine. A MINE!!!!!!!1111ONEONE" Him getting so excited over something so boring makes him sound like a crazy individual, characteristic that strongly and unrealistically contrast his usually calm and careful nature.
-try not to make the levels and the story clash:if you're in a human's digestive system, how comes it doesn't look like a human's digestive system? You could either make it more realistic (at least on the minimap) or change it to something abstract like a Cyberdemon's digestive system.
-try playing your own levels before releasing them to make sure there's nothing wrong;
-try to have a team of beta-testers before releasing a wad: a typical team would consist of a hardcore player, a casual player, a new player, an oldschool wad fan and (if necessary) a newschool wad fan;
-try to take ideas from good mappers or map-packs: Alien Vendetta, Deus Vult, Hell Ground should be a few great sources of inspiration.
Ok, that's all for now! Hope you can make use of it. Good luck with mapping, and I hope to see some other maps from you soon! You have great potential, and in spite of all the criticism, I really loved DTS-T! You did a great job with it overall!