-- iOS devices blowing their asset layout and 'Othering' out is a Real Problem that used to happen far too often. The only fix beyond a backup+restore is to remove and re-add photos and music. If that doesn't work? Time to restore up to 60GB over USB2! Whee. Good luck explaining this to mom.
-- The built in Mac applications and frameworks are frightfully poor - it's unacceptable from a company that prides itself on quality. SyncServices is still a flaming travesty, Mail.app spontaneously corrupts messages and passwords, Spotlight can die in twenty different ways, iCal is a UI disaster, Address Book has completely broken sync options... the list goes on and on and on. Of all of these, I think Mail is the absolute worst. Three total rewrites and it's still neurotic on a good day.
-- iPhoto is goddamn slow. No matter what, no matter where, no matter when.
iOS is an order of magnitude more usable for two orders of magnitude more people with an order of magnitude fewer issues and two orders of magnitude fewer things to go wrong that makes an order of magnitude more money for them. So I think that's where he Lion's share (haha) of Apple's QA is spent. Sadly, I fear OS X will never receive that same level of care.