What isn't a beginner bike?

No matter what bike you got there always seems to be a dude asking you when you're gonna get a "real" bike. For some people its about displacement, for others it's about horsepower, but regardless of what it's about I see people calling 500-750cc bikes beginner bikes. Things with 50, 70, 80 horsepower being called beginner bikes.

What exactly makes a bike not a beginner bike?