I wanted to provide a very quick example of how the user interface has changed from OS X Mavericks and OS X Yosemite, and hope to illustrate why Yosemite's interface is a step backwards in certain regards.
Here's the home page of my website loaded up in Safari on both Mavericks (front), and Yosemite (behind). These observations are not specific to Safari.
You can see the Safari toolbar in Yosemite is much flatter. But what strikes me the most is how user interface elements that were previously embedded *into* the interface material now rise above, such as the text boxes. This makes it more difficult to distinguish between regular buttons and text boxes. It's a big step backwards, particulary for users that are learning the Mac for the first time. There needs to be better visual separation between such elements, in my humble opinion. I really, really hope Apple sees this and fixes it.