Paul Horowitz, writing for OSXDaily:
The newest version of Safari in OS X Yosemite defaults to only displaying the domain name of the website you are visiting, rather than the complete URL that many users have long been accustomed to seeing.
If you’d like to change back to how most web browsers have behaved for twenty+ years and display the complete website URL, you can make a quick settings change on your Mac within Safari to do this.
This was the first thing I tweaked after upgrading to Yosemite. Thankfully Apple made it easy to change. I wish there was a similar setting for Mobile Safari on iOS.