Besides the predictive power as a way to measure "realness", I would add persistence. A car in my dream is less real than a car before me when I cross a street in my daily life, in the sense that it persists more in my mind.

Awesome post. I think that lesswrong asks good questions to make you more rational. But in a complicated fashion. Although I am a newcomer, I think that there is a waste of time focusing on the rhetoric instead of practical problems. I have the feeling that lesswrong will be giving me some insights about the kinds of "algorithms" that my mind will need but, indeed I need to apply those "algorithms" in the real space in order to fully implement them...