Basically, Inception asks us if matter matters. If we change our definition of real, we can change our reality. Yeah, it's deep.
Cobb changes Mal's perception just like the extractor team messes with Fischer's perception. In the dream, Fischer's father seems very real, much like Mal can be real if Cobb lets her. "Real" is just a matter of perspective.
Like the people dreaming in Yusuf's basement, the world we think is real is the world that matters most to us: it doesn't have anything to do with physicality.
Questions About Versions of Reality
If you could dream whatever you wanted to dream, how much would you dream? Would you spend all your time sleeping or would you try and limit your time in your dreamworld?
How is a shared dream different from a shared virtual world? Maybe video games in a number of years will be like what Nolan created in Inception. How would you evaluate that kind or reality?
The big question most people ask is: was Cobb dreaming? Does it matter?
Chew on This
Even in the physical world, our experience is completely in our own minds. We only think it's a physical reality because of what we think we know about the things we see.
Each of us has a moral obligation to other people, and more consciousnesses make a reality more real. That's why dreams will never count as "real life."