There's this Wi-Fi strength indicator, long overdue, in the dock.
And when a battery is close to end of life, the OS will report that, too. (To clarify, not only a dead battery, but one that will fail to hold its charge well.)
Automatic spelling correction in textedit.
Among other speed enhancements, boot and shut down twice as fast as in OS X Leopard.
Of course, these are really trivial next to things like multicore and GPU processing. But refined details are something I appreciate in an operating system.