Apple is planning a more restrained approach to the look and feel of OS X 10.9, with changes geared towards power-users rather than entirely overhauled functionality.
Word from 9to5mac is that the new OS X, internally codenamed “Cabernet,” is centred mostly around improvements to the Finder. There will now be tabbed browsing within the Finder and the ability to tag things to make searching your computer easier.
Safari will also be given a redesign, with an improved back end that should speed up the rate at which pages load. Multiple monitor support for different full-screen apps will also be unveiled.
There are rumours of built-in Siri functionality as well, but this is yet to be confirmed. Also possibly making its way over from iOS is the app drawer for multi-tasking.
Previous Apple OS X software updates have nearly always been released in the summer, so we wouldn't be surprised if the same thing happened here.