Upon upgrading to 1.8, a witch hut fails to continue spawning witches.

Download the attached world, and open it in 1.7. Fly away from the two iron-block spawning platforms, and you'll find that only witches spawn on them, and nothing else. This occurs because there was originally a witch hut in that spot, and I removed the terrain around it.

Now exit the world and load it up in 14w29b. You'll find that other hostile mobs spawn on the platform, and not only witches.

This suggests that the structure has been removed (or altered somehow) to allow other mobs to spawn.