Some map making rules to keep in mind.
Repetition is the number one evil in an SP map.
I just played some of the maps being worked on in FreeDoom and every once in awhile I'd get real excited by something in the levels and then annoyed. The texturing - PETAL TO THE METAL! Jeezus - easy on the metal FFS! Sure, id maps were metal heavy, but it didn't have the overbearing effect these maps had on me, particluarly MAP03. I really liked the lighting around that corner with the cement texture. But was horrified again when it went back to more dark, drab, boring metal.
And what's with the maps starting out looking a lot like the originals and then branching to be something else? I personally think this is a BAD idea. The levels should be totally original imo, but have them still make sense to the original level name, but in an original way. I did like the crate section on MAP06, though; even if it seemed a bit randomly placed and purposeless in the map.
The other thing about repetition. MAZES SUCK. period. Now let me just say that the maze in E1M2 is small and kept interesting by several secrets. In MAP06 - I loved coming to the lift that takes you down and looking out at the nuke thing below. Much better than the id version imo. However that maze below, as cool as it looked, was annoying as hell to run around in! Serious turn off. Also, it reminded me of Half Life :-/. I think that hallway thing should stay - *******ely, but needs to be SHORTENED to just a passage leading to something new.
Of course all of this requires more work and is more time consuming and more difficult, but it will make the maps infitately better. It keeps the player interested. At least for me.
Now Base style maps that are like a grid are the ultimate evil. Detailed, ritzy, uncreative stolid hallways. Nothing is more boring, dull and unappetizing than these. You know what I'm talking about and if you don't, well, I can't think of any examples because whenever I come across one of these maps I immediately cringe in horror and never touch it again. :P
What if software archaeologists unearthed levels from the original DooM 1 that had somehow been lost and never made it into the original release of DooM? Project Doom the way id did explores what those levels might look like.