Ugh, I really hope this doesn't inspire a whole dream-focussed genre for the next couple of years. That I see getting old very fast.

I think the question of whether the top level world was a dream was well handled, actually - it wasn't discussed explicitly, it wasn't presented as a Great Reveal, it was just left as a possibility. I think it would have been much cheaper to go 'tada! Look, this was all a dream too!' So I'm rather glad that impulse was resisted.