various gases would form and probably blow us all up to heck...? :P

Someone would find a way to sell advertising on the interior surface.

Nothing .. indeed that would explain why the mass of the earth isn't consistent with its volume.

nothing... the only thing that i could think would happen would be that the crust would collapse and you said assuming it wouldnt

"Hollow Earth is a belief that the planet Earth has a hollow interior and, possibly, a habitable inner surface. The hypothesis of a Hollow Earth has long been contradicted by overwhelming evidence as well as by the modern understanding of planet formation, and the scientific community now dismisses the notion as pseudoscience." "Someone on the inside of a hollow Earth would not experience an outward pull and could not stand on the inner surface; rather, the theory of gravity implies that a person on the inside would be nearly weightless. This was first shown by Newton, whose shell theorem mathematically predicts a gravitational force of zero everywhere inside a spherically symmetric hollow shell of matter, regardless of the shell's thickness. A tiny gravitational force would arise from the fact that the Earth does not have a perfectly symmetrical spherical shape, and also from forces due to masses such as the Moon which do not form part of the spherical shell. The centrifugal force from the Earth's rotation would pull a person (on the inner surface) outwards, but even at the equator this is only 1/300 of ordinary Earth gravity. The mass of the planet also indicates that the hollow Earth theory is unfeasible. Should the Earth be largely hollow, its mass would be much lower and thus its gravity on the outer surface would be much lower than it currently is." Source and further information: http://en.wikipedia.org/wiki/Hollow_earth