but doesn't work too well. What are the rules for spawning mobs now anyway? It needs to be dark, what else?
That video shows an exp farm being built in a superflat world, which doesn't normally have any caves/ravines/etc.
There's a hostile mob limit (79 or so) so if you have 79 mobs within 128 block radius of you already sitting around in caves and whatnot, then no mobs will show up in your exp farm until one or more of them despawns (average lifetime 30 seconds.) Even then, the game is likely to choose some dark spot in all the caves/ravines/etc instead of your darkened box (exp farm.)
So - IMPORTANT - try to get FAR away from ANY other surface where mobs can spawn.
2 ways of doing this:
1) Build high in the air - above the ocean at y>160 is easiest to get the 128 block distance you need.
2) Light up all the caves/ravines/etc with torches (AND the surface.) But the volume of 3 million blocks is a lot of volume to explore and light up!
Mobs won't spawn closer than 24 blocks to you.
Mobs won't move around [much] more than 32 blocks from you.
Mobs won't spawn on lower half slabs and some other non solid blocks (plain cobblestone is fine of course.)
Mobs will spawn only if light level on the block is less than 8 (AND, the lower the light level, the better)
Mobs won't spawn if spawning would make them collide with anything (exp farm should have 2+ blocks free vertical space.)
Mobs spawn better on a flat surface (reason being that the entire pack (4) tries to spawn at the same Y, and they are not allowed to spawn in midair or inside a block.)