Confirmed for 1.8.2-pre1 in Win7x64. Beacons need a revamp, imo. The whole 'path to sky' requirement seems unnecessary. Overhead path requirements should be more or less entirely removed in the Nether, in any case.
I can confirm this is present in 1.8.1. It appears to be a gui-related issue. You can cycle the button to peaceful, but hostile mobs will not despawn.
I am experiencing this issue on Win7x64 with Java 7.65, and minecraft 14w29b
I only recall seeing this since bug they added the feature that made the game 'more difficult' the longer you spend in an area. There was never a satisfactory answer for how the spawn algorithm was changed with this feature, but it is entirely possible this problem with spawn rates may be related. Just a thought.
I experienced this problem in 1.7.4, using 2GB and then 4GB memory allocation. I have been running 1.7.5 without any command line switches and my spawn rates seem to be back to normal; my mob farms are operating at nominal production rates and I am getting mobbed in the Nether again.
I've experienced the same issue since 1.6.4. I use Vanilla MC. On one world, I found that an unrestrained villager breeding cell had produced about 600 mobs in a 2x2 space. Once I killed them all, this problem went away. However, it is still present in other worlds that do not have mob breeders, so I can't account for it.
After downloading 14w02c, I can confirm that this is still an issue for me. Anecdotally, I've noticed that I seem to get normal spawn rates when I am within my spawn chunks.
Seed: 7934725885111696041
What tool do you use to view the NBT data in-game?