IDG Answers is a community of experts who are passionate about technology. Ask a question or answer one below.
The differences that will be immediately apparent from a visual standpoint are a change of font, which has a more significant impact that it sounds like it would, and a revised UI with some gee-whiz features like translucent windows a la Windows Vista. Icon design will be more unified across iOS and OS X.
The biggest functional upgrade is probably the integration with iOS with what Apple called Continuity. This will become more apparent when iOS 8 and Yosemite are both in the wild. This will tightly integrated the functions between Mac and iPhone/iPad, so that there is little distinction for many actions such as messaging and phone calls (as along as your iPhone is nearby). I think this is really a nice step forward, continuing what Mavericks started and further refining it. We are seeing this across platforms, with Windows 8 being used across PC and mobile platforms and, to a lesser extend, the integration of Android and Chrome OS