I was one of the early (I think my 1st installed OS X version was developer release 4) adapters of OS X. And I very rarely thought back of OS 9/8/7/6. All the new thrills of the new OS made you forget about the good things of the classic OS’s.
This blogentry however summarises the most missed features of the old classic OS.
I personally miss feature #6. Tabbed folders the most.
update 2014-11-22: OS X has tabbed finder windows now again for years… never use them ;)