Jason Snell's Hands-On of Yosemite →

After what I've seen from Yosemite so far (I haven't been able to try it yet), I agree with what Jason Snell writes in his hands-on of OS X Yosemite:

"Some of the design decisions in this first developer release seem a little unfinished, which is fair enough—the OS itself is unfinished! I’d like to see more consistency when it comes to the way stoplight buttons and title bars are displayed, and I worry about the increased clutter and decrease in clickable areas for dragging windows. I’m also a bit concerned about the loss of window titles in apps where they impart real information. It’ll be interesting to see how Apple designs its individual built-in apps, and what decisions third-party Mac developers make on this front."

Some decisions with subtle design changes and differences don't seem to be final yet. There are even inconsistencies in UI elements in Apple's apps, but it's still in beta after all.
I'm looking forward to using the new design, and after seeing more and more of Yosemite, Mavericks is starting to look really old and a little rough around the edges.