A very efficient Gold farm could be made in the Nether Above the Bedrock and just below the build limit. TONS of Zombie Pigmen would spawn due to nothing hindering the spawn limit from surrounding areas.

However, nothing spawns above the bedrock anymore in 14w27b. (See Image) My Iron Golums used to be VERY busy killing Pigmen. Now, as you can see, they are very bored, indeed.

I am in Hard Mode in the image.

Edit: Actually, NOTHING is spawning in the Nether, above OR below the bedrock! I went ALL over the Nether and I didn't see a single mob. The Blaze Spawners (cages) are working, but nothing is spawning in the regular spaces. I tried toggling difficulties, exiting and reentering the nether, logging off and on... Nothing!