The game crashed a short time after entering the overworld (I logged on in the Nether). This happened in both the 1.8.1-pre5 and 1.8.1 version. I attached both crash dumps for reference.
It seems to be related to a moving poppy in the spawn chunks. I do have an iron golem farm and a dispenser shooting poppys into a nether portal to keep the chunks loaded.
Environment
OS: Windows 7 (ver 6.1, arch amd64)
Java: 1.8.0_20 (by Oracle Corporation)
Launcher: Minecraft Launcher 1.5.3 (bootstrap 5)
Minecraft: 1.8.1 (updated Mon Nov 24 15:13:31 CET 2014)
Duplicate of MC-73866