1) FaceTime for Mac isn’t designed well for computers that are shared by two or more people.
To use it, you sign in with your Apple ID, in the form of an email address. That’s the ID that people “call” from another FaceTime device. But only one account can be signed in per computer at a time. So if you share a computer with someone, only one of you can be waiting for FaceTime calls at a time.
This isn’t a problem with FaceTime for the iPhone or iPod touch, because people don’t usually share those. But people share Macs all the time. So Apple will need to figure out a way for multiple accounts to share one FaceTime-for-Mac app.
While they’re at it, they might figure out ways to make sharing an iPad better.
2) FaceTime rings on every Mac you own (and have signed into), even if FaceTime isn’t running.
That means your home computer may ring if you’re at work, and vice-versa. Not a big problem, but could be annoying.