I agree with the hole in glados' chamber not showing other floors, and also: You fall a LONG way down, you couldnt have gone all that way back up in such a short time.
The potatos stayed intact for 230 years? No way. Even the big potato plant (by Chell, I love that easter egg) would have died. I say that something happened which tore through the centre (the reactor blowing or something?) which sapped the colour out of everything (radiation could screw that...