Well there's actually a lot I learned from that. It's a culmination of watching KingDime's history of the WAD as well as hearing Tarnsman and others talk.
Doom is really a unique thing, but it needs together like a C4 and a Detonator, cookies and milk.
DOOM's ingenius and amazing chess strategy like gameplay of not just run and gun but run, strategize, improvise and wielding weapons that are NOT the least bit underpowered this side of the handgun.
It has to go together with GOOD maps really good maps compliment the gameplay well. There's a reason people choose Ultimate Doom over Doom 2's. There's a good reason a lot prefer Plutonia to TNT. TNT had a lot of good intentions and ideas but didn't quite click like Plutonia's maps did to me.
There could be HUGE and there is HUGE maps that are either in fan made Megawads or even official wads like Final Doom or whatever and they could just be..bad. Really bad. There's got to be something planned with it to make it good. Not just big for the sake of big.
Look at Community Chest Map 29.
http://www.youtube.com/watch?v=TZp9cT6PDh0
so AMAZING looking at the beginning, but it's honestly a clusterfuck of a mess. Looks amazing on the outside though.
A lot of people also think that bigger maps = more difficult, yeah that's true if you want to turn it into a slaughtermap practically. Some of the most difficult maps aren't even that big like a GREAT example is E4M1 or E4M2 or even Underhalls in Doom II. They're basic as hell and got the difficulty.
So in all with this rambling on, it could be in the old style of doom with mazes and things and still be garbage cause they're not designed well, they could be a clusterfuck but a pretty one or a ugly rat maze.
Maybe that's why there's so many 'O' of destruction/Dead Simple/Underhall/Entryway clones.
Wrack has that problem of the areas almost all looking the same and it's easy to get lost or confused and there's no automap. Least Doom has distinction where you can tell different areas apart.
Lot of thought and ingenuity goes into map making, and it'll show in a lot of the good wads listed by the OP and ViewtifulJC.
Heck even new game levels can be done well, Bioshock 1 had some pretty good level layout if I remember right.