The MySpace to Facebook transition shows that exactly this happened. I was in high school at the time, and I definitely remember people mentioning having both a MySpace and Facebook account, and wanting to get people to switch over to Facebook. I don't mean "tech people", I mean normal folk.
As for compelling features, I think Circles is a killer. When your relatives are on Facebook, a good portion of your personality has to leave Facebook. You have to think about what you're going to say, and generally stick to the blandest possible expression of who you are. If Google+ enables people to say what they want to who they want, it will be a hit.
I still dunno if Google+ will be a success, but if it is, I suspect it will be more due to the actions of early adopters than to Google.
Any guesses as to how long it takes for Facebook to copy G+ circles?
Also the "average" person won't maintain 2 social profiles and will gravitate to where the users already are. If Google wants to compete in social it's need to do more than just "sharing". They must have features that people need to use. These features may come. I don't know what that is? Deep personalized search integration? Beat Facebook to their inevitable Skype/video integration? time will tell.