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!
I think the best decision is to adapt the iPhone OS interface paradigm, but not relegate the Mac OS X technology to benefit the iPhone OS X software stack. The iPhone OS simply does not takes advantage of the hardware of a personal computer.
How useful is the iPhone OS 4 multitasking API on a computer with 8GB of RAM/nearly unlimited virtual memory over a SSD drive and a Sandy Bridge CPU?
For OS X 10.7+, Apple needs to introduce the next things:
-Up to time OpenGL implementation.
-Security technologies as ASLR, to improve the system security.
-Start taking advantage of the Mach microkernel, or, if it's a bit outdated, replace it with a open source modern one. (This maybe a feature most suitable for a major new version, like Mac OX XI)
-A built-in system to remove the additional software installed by some applications.
-A new high reliability/performance optional filesystem like ZFS.