Python 3 killer feature - asyncio
I've been really conservative with the whole py2 -> py3 migration (shiny new langs don't seem to be my thing), but one feature that finally makes it worth the effort is well-integrated - by now (Python-3.5 with its "async" and "await" statements) - asyncio eventloop framework.
Basically, it's a twisted core, including eventloop hooked into standard socket/stream ops, sane futures implementation, all the Transports/Protocols/Tasks base classes and such concepts, standardized right there in Python's stdlib.
On one hand, baking this stuff into language core seem to be somewhat backwards, but I think it's actually really smart thing to do - not only it eliminates whole "tech zoo" problem nodejs ecosystem has, but also gets rid of "require huge twisted blob or write my own half-assed eventloop base" that pops-up in every second script, even the most trivial ones.
Makes it worth starting any py script with py3 shebang for me, at last \o/