Traded the 2010 for a 2015, and my wife says, "Where is the direction indicator in the corner of the mirror?" Doesn't seem like a huge cost savings to do away with a handy feature... Also, they rotated the steering wheel thumb controls 90 degrees, so they go top to bottom instead of side to side. This one I don't mind, in fact I think it's an improvement. Any other minor differences that I don't know about yet?
The compass is a handy feature. It has sometimes saved me dozens of miles of going in the wrong direction. Most vehicles have it now, but if it's not there, its not an expensive accessory to add on.
I have a 2010, and it doesn't have a compass indicator in the mirror. Of course, I have the nav system (which has a compass on it), and the mirror is the HomeLink variety (I'm not sure if they use a HomeLink mirror with the compass display or not). A few Prius variants put a rear-view camera display in the mirror. It wouldn't surprise me if that were incompatible with the compass too. Toyota.com tells me that the Three has navigation standard now, so that's what my money is on - you still have a compass display, you just need to look about a foot and a half lower.
