I built a mob farm but nothing ever spawns in it. I'm guessing the caves somewhere below hog all the spawns so the mob cap is always full.
I did dig down and find some caves and slapped some torches down, but that's an exercise in futility with how expansive the caves are. And there's multiple layers, too. Mobs did spawn down there so it's not bugged and not on peaceful. Oh well. At least I came out with diamonds.
But I have to get the mob farm working somehow or I'll never get villagers. Maybe if I build it high enough so everything in the caves despawns?