I miss the spirit of the classic Mac OS. Customizability, while lacking in comparison to Linux and Windows, was light years ahead of Aqua-based Mac OS X. And I still feel as though the Apple menu was better than the Dock/Spotlight; I don't know why they killed the Rhapsody-based OS X's version of the Apple menu. Tabbed windows were great, and I used them often, and I liked the implementation of labels over OS X's version.
In my opinion, Mac OS 8.6 was probably the best mix of features/resource utilization/stability though I still prefer System 7.6. All told, the classic Mac OS had a lot more character than Mac OS X does today. Plus, classic Mac OS is the only version that will natively support my Quicktake 200. I suppose I could use Windows for it, but... 🙄