fryke :
The open source part of OS X is old, that's true (base on FreeBSD). But most problems concern the interface : nobody complain about the multitask, or about the memory handling. Speed, logout with applications open ... all that are on the not opens source part, and this one is not as old as rhapsody.
Another feature that I would like to seed is virtual screens : the ability to many totally independent screens, that you can choose, for example by hitting ctrl-option-F1/F2/F3 ... On one screen, you can develop a web page, an you can test it on another screen ... way easier than having many layers of windows I think.