While I would love power and stability improvements all around, and under the hood improvements to drivers and APIs, I'd also love to see the integration between iOS and OS X finally done properly and completed. For instance:
Unified Clock/Alarm across all OS X and iOS devices.
Unified Do Not Disturb across... (plus allow me to set DND for specific events in my Calendar in addition to what they already have).
Unified Book/PDF reader across... (and synced through iCloud).
Unified iWork/iLife across... (i.e. include feature parity to the iOS apps so the syncing works properly).
Unified Safari across... (i.e. add in extensions to mobile Safari so that I can finally have the identical experience on either platform).
Plus: they really should finish the enhancements to Mission Control, regarding the managing of full-screen apps in single and multiple external displays, + fully restoring the order/arrangement upon reboots.
Plus Plus: Fix Time Machine once and for all. This is supposed to restore your entire system, so have it actually restore the system 100%, including the hidden partition so that we don't lose functionality like find my mac, requiring a re-installation of OS X after restoring through time machine.
Excellent points, agreed on all