I am reluctant to play (or flat-out refuse to play) the following:
-Plutonia MAP11 and other Arch-Vile mazes
-Any/all Icon of Sin maps - even the one I made in Scourge
-Big slaughter maps - I've never played Sunder, Slaughterfest, New Gothic or most of Time of Death's maps, for examples, whilst I have happily played through AV, Plutonia 2, SoD, HR, HR2 and Kama Sutra (just to give an idea of where I draw the line). The second I see a big grid of monsters in a large, open area, I've lost interest
-Joke WADs - I've given some a go, thought they were alright (Wills made some good ones) and then seen a huge number of things (Terry's and Doomguy2000's stuff, for example) that I flat-out refuse to even give a go
-Maps that rely on speed running tricks. I can live with strafe running, but when I have to try and get glides and stuff, I generally lose interest
-HeXen and Heretic - The games themselves didn't appeal to me too much, even though I've beaten them both once in their entirety, but I am yet to ever want to play a .WAD for them all the way through, IIRC
Wow, we're like, the same guy. The biggest difference is that I only play coop so I'll play (for a while, at least) almost anything fun. I never have played a joke wad. I'll play Plutonia MAP11 because I just have to suffer through it and maybe half a dozen deaths. Same with any map that's too hard. I'll usually make sure cheats are enabled for Icon of Sin maps.
Which reminds me.. does MAP30 have to end with an Icon of Sin? Could it not just end with a regular switch? I've played through probably a couple dozen megawads now and every single one of them has Romero's head on a stick at the end.
The only thing I truly can't stand is consistently not having any ammo. I better have plenty of ammo for at least 1 gun, preferably shotgun. The SSG is tough as hell but fighting a cyberdemon or a room full of big monsters with one is as ammo-restricted as I ever want to be. That's why we always play with "keep ammo after dying" enabled. It kind of cheapens death but not to the point where we sacrifice lives.
Also, after playing the first few maps of Speed of Doom the other night, I hope I never see an instant teleporting monster again. The concept makes absolutely no sense, and SoD does it at what seems to be completely random times. Stupid and annoying.