The problem is that reasonable unicode support is a non-issue in many areas were python had a strong competitive advantage over other languages (e.g. data analysis, scientific python).
I don't know much about unicode myself, but some people I trust like Armin Ronacher have showed how some python3 are still heavily broken w.r.t unicode (http://lucumr.pocoo.org/2013/7/2/the-updated-guide-to-unicod...).
So, Python 3's killer feature is reasonable Unicode support.