- clean Unicode support which prevents mistakes which used to bite us in production
- native async syntax, compatible with Tornado
- asynchronous generators (yield from a coroutine!)
- pathlib and os.scandir
- type annotations
- matrix multiplication
- chained exceptions
- faster dicts, ordered by default
So many useful features, and with the latest Python 3 releases porting has become really easy thanks to improved compatibility with the old syntax.
I think the flip around - ~30% adoption of a newest version at 1-2 years in - especially one that breaks backwards compatibility isn't bad.
I've started numerous python projects at work over the last year all on 2.7. I think we just started the last one. We're finally ready to jump to 3.
If async and this had been in Py3 from the start, they could've done a far better argument saying "hey, we broke all this stuff, but you get asyinc I/O and faster dicts" (a note of faster dicts: since many classes and language mechanisms use dicts, it should have an encompassing effect on all of python performance -- maybe not 30% or 40% faster, but a few % all around, which is kind of what micro benchmarks are showing with 3.6 betas)