The dashboard is a lot better. Pins are no longer hidden away. Learn the voice commands and you can do stuff very, very easily. You can of course do things the traditional way and use your controller.
Smartglass has also had a MAJOR improvement. There's a whole new app for iOS, Android and Windows phone. Any time you need to type something in, whether it be to redeem a 25-digit code (Kinect can read QR codes btw - makes for redeeming codes extremely quick) or messaging someone, you can quickly and easily use the keyboard on your phone. Can't remember if you could do this on the 360 smartglass app but it seems to be much easier on the One smartglass app.
Also it's very quick. It literally is as quick as the demo videos show. I'm able to go from Forza Motorsport 5 (digital version) to whatever is in the disc tray with a simple "Go to" command. It's the bone idler's dream!
Whether these make it worth the upgrade is down to personal opinion.
Also dat bluray.