After editing in a few diamond tools and torches, as well as taking advantage of some of the blocks laying around, I believe I see the general ideas behind this trap.
Overall it seems as if the spawning area to non-spawning area ratio is considerably smaller than many mob traps out there. However, the spawning area is also completely horizontal, which seems to be ideal now that the chunk 0 bug has been fixed. Mobs also are killed much faster than in most other traps, which allows spawn rates to be considerably higher than many other traps (assuming both are consistently at the spawn limit).
Interesting ideas, and obviously effective. I wonder how feasible it is to use the same concepts when building one without hacks. I'm fairly certain that this trap can only be scaled down to a certain point effectively. Once the trap is on a scale such that the mob limit is not consistently at the cap within the trap, I suspect the low spawn to non-spawning area ratio will make this trap significantly less effective than those commonly seen.
Yeah I've had the same thoughts, and I honestly think that you need to have it built as low as possible on the map, and you need to light all of the caves within around 80-100 blocks around you, above you. If you can do that then a smaller version should give a good amount still, although as you said it'll be significantly less, seeing as how I did everything possible just to take full advantage of spawn rules.
I was wondering too if you built it as high as possible though, then there would be less area below you to cover as far as lighting goes, even more if it's built on a smaller scale over an ocean.
I hope someone can take the idea behind it though and successfully build something that works nice and can be done fairly easy, legit.
What material did you find to have the best spawn rates?
As far as where the mobs spawn? Anything that you'd naturally find underground I just guessed would work. I used stone for the floors with some Iron blocks and cobblestone for the walls. Stone or Iron for the ceiling, anything that light wouldn't 'pass through' pretty much works. As far as items, arrows and bones are killing me, sulfur has a nice income though. The way the lava traps work though is nice in my own opinion because it seems to burn some silk that spiders drop when they die, and I don't want much silk anyways (in a real, legit gameworld - which this will never be in lulz).