What I've found from my quick play with Yosemite:
Radio buttons, drop down menus and menu tabs all look a lot different. Here is a screen that displays all of them, I find radio buttons especially weird but I'm sure I'll get used to them:
Calendar colours are a lot brighter, though thankfully they don't translate as vividly into the app unless the event is selected:
The dock 'fills up' when minimising a window:
Spotlight does currency conversion (I know they showed distance conversion in the Keynote, but I was wondering whether currency would work since it relies on fluctuating exchange rates):
Holding down Command while selecting something in Spotlight shows its file path:
Full screen Finder's a bit weird with the sidebar:
Bug: iMessage displays two flashing text cursors when you first compose a message:
Only other bugs I've found is that something blurred behind Safari flashes while the status bar is enabled, the battery indicator was green when fully charged but stuck at about 30%, and that no dock icons rendered until I restarted it. Those are all fairly minor issues and it seems pretty stable, though quite a few apps still don't support it so I wouldn't recommend it on your main machine (my Yosemite install's on a 500GB USB 2.0 drive, which surprisingly runs quite well.)