Normally, Endermen are supposed to teleport away and not be aggressive towards you if they touch water or rain. I stared at an Enderman from afar during a rainstorm in survival 13w39a. It began teleporting everywhere, trying to find a dry spot but couldn't like it normally would. However, it'd continue to be aggressive towards me, teleporting to me occasionally and smacking the crap out of me over 10 seconds until I died.

I'm not sure if this is a 'feature' to make endermen more difficult, but seeing as Mojang changed zombie AI in 1.7 snapshots from what it was in 1.6, I doubt it.