FWIW, I just founded a new startup and we're using Python3. No major problems so far, and I find it much more pleasant to work with than Python2.5-2.7 (which I used in my last startup, and at Google). Dictionary comprehensions are great, keyword-only arguments have been necessary in a couple cases, Unicode's not a total mess, I like how the standard library naming & packaging has been cleaned up, and I suspect I'll love asyncio when the time comes to use it. The only library we wanted which doesn't support it is Splunk, and that was Splunk's loss (we just switched to PaperTrail).

There's an awful lot of FUD and hate for Python3 on Hacker News, but I've found it's great for a green-field project. And folks with 500KLOC existing systems should know that their technology choices are basically locked in time to when they started the project, anyway.