Well, there was also the Lion period where you literally couldn't use fullscreen and multi displays... despite previous versions supporting multiple displays perfectly. Took them until Mavericks to sort that out.

I've had lots of issues with Mail.app recently too. Especially with older IMAP servers. Then there are also issues with the systemwide accounts.. ugh. I've literally filed over 100 bugs in Radar in the past 2 years vs a handful before that.