This is a carry over from some of the later 4.x builds, but if you have more than one bluetooth device paired to a phone, the phone will randomly disconnect from the currently paired device and try to connect to one of the other ones on the list.

Video black levels are now set far too high, especially noticeable on the Nexus Player in Google apps like Play Movies and when casting Photos or mobile screen. Everything looks washed out, and blacks look grey.

Kodi is still fine though, as is the regular Chromecast.

Edit: also it was confirmed that the recently fixed Google Cast bug had nothing to do with the Marshmallow update.

Not a bug because I believe it was intentionally removed but....

Home Launchers like HALauncher can no longer be set as default.

Fishing for bugs in a forum? This is the laziest journalism I've ever seen.