My experience over the past year has been that 80% of what I need for new projects is already available on 3 (with the remaining 20% being swappable for other stuff), and that asyncio is a massive (if controversial) boost in performance for some things.

I do wish that some of the gevent/Cython stuff was 100% compatible on occasion, but it's manageable.

That said, 2.7 is still the default in many environments, and we're likely to have to live with it for 12+ years.