As a human factors guy, and I know a number of people who echo this sentiment, I feel that while OS X may be built on some robust technology (Quartz, QuickTime etc etc), it is a usability nightmare. From accidentally launching docked apps, to constantly re-sizing windows to annoyingly small file select dialogs to a lack of creative 'Microsoft Notes / Courier' like next gen interface technology etc etc. (I know many who MUCH prefer the snappy spacious nature of OS 9 - myself included.) Stability is the only real benefit of OS X.
As I stated on my blog over a year ago, Apple should focus on turning 'iPhone' OS into their primary OS - with tight cloud based architecture and semantic file storage and sharing technology. Using creative apps on iPhone OS is, no matter their initial simplicity, an order of magnitude more intuitive and fun than any OS X equivalent.
I for one would pay big money for a multi-touch 27" 'iMac Touch' with an MS Courier like GUI for spontaneous accurate creativity and productivity.
No matter what Apple have invested in OS X, they have to consider the future. It is just not good enough from a usability angle.
So toast me!