For me it's if the screenshots have a slick, detailed design with good attention to detail and lighting, especially with almost purely stock textures, I just can't get enough of these textures, they're like old friends I've known for years, and keep coming back for them because they feel me with intense nostalgia that can still surprise you, depending on how they can be used for many new tricks and interpretations, such as the burnt toast in DBP37 MAP02. Bonus points if it's vanilla DOOM2.EXE compatible, these are always impressive feats, feels like these demoscenes back in the 90's that could do some impressive stuff at the time, I personally wasn't really there to experience demoscene, but if I was; I think this is how it would feel like.
If it requires chex.wad or heretic.wad, with Chex I can be guaranteed a quick and easy experience, and with Heretic you can get very creative with the inventory system.
Themes such as city, and interesting ones like in DBPs can also pull me in.
I generally don't care for any technical limitations unless I design them.
OTEX is ok, but feels too sterile and clean for me, I much prefer the roughness of the stock textures, as well as for mapping OTEX is just too cluttered and overwhelming when I want to choose a texture.
ZDoom requirements deter me from downloading wads too, because wads that require it usually aren't fun to play, I'm guessing it's trauma from playing KDIZD, ZPACK, Community Is Falling 3, and DTS-T (The Adventures Of Square is awesome though), which are completely unbalanced and/or confusing, and/or plain boring to play, but I don't mind Zandronum however, because it has much more fun exclusive wads/PKs that I've played since Skulltag days were over (I WAS THERE!!), such as Zombie Horde, Deathrun, Complex Doom, Barracks Zone, Jumpmaze, Jumpix and StupidLMS.