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 updated my graphics driver, had the system hardware checked out. Problem still persists. On one world, I found that there was unchecked villager breeding. About 600 mobs were in a 2x2 area. Once I killed them, the problem seemed to disappear from that world. However, it is still an issue in my other worlds, and those do not have villagers breeding.
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.
Crash report attached. This issue seems to get worse if Minecraft is opened and closed repeatedly (possible memory leakage?).
Seed: 7934725885111696041
What tool do you use to view the NBT data in-game?