Gave a couple of classic WADs a go:
Remember when you were a kid and you were drawing a house, and it would be disproportionally huge? Eternal Doom is like that. Long-ass corridors and huge chambers, but then an almost anal approach to key- and switch-hunting. While there are some clever sections here, I gave up at about Map08 or something, I just wasn't having fun with this one.
Requiem is considered a classic because it did some truly neat things with the Doom engine back in the 'early days'. Unfortunately it has aged badly. The order of levels is completely out of whack, The build-up in enemy and weapon progression is too slow, and what at first appears to be connective levels (the start of the next level sharing traits with the end of the previous one) quickly falls apart. I use a teleport to end one hell-ish level, only to find myself standing on a toilet in the next one! The only challenge brought forth by the maps is having to force the player to look up a walkthrough to progress through the levels.
You want to create a convoluted series of events so that players progress through your creation? That's nice, but check to see if you're in the right game first. Doom is not the venue for hunting for hidden buttons... even if you have to shoot it on a pillar across a room.
And ironically enough, the only time I was short on ammo was during Map29 - and when I progress to Map30 with my low ammo supplies, I'm greeted by a Super Shotgun and little more before being pitted against the Icon of Sin in a funnel-esque design - meaning that all the monsters it spawns are placed between it and you in a cramped space. Interesting challenge, but by this point I just wasn't having it.
I don't recommend either WAD for playing, but both WADs are almost quintessential resources for budding level designers looking to pick up a few tips and tricks, so there's that.