It seems the vampiric mouse has been wallflowering this conversation:
http://www.enworld.org/forum/columns/310139-plot-not-four-letter-word.html
Needless to say, I agree with pretty much all his points except one - the continuum is not sandbox <---> railroad, it's sandbox <---> linear. Again, it's ridiculously easy to railroad in a sandbox campaign.
I agree of course - APs are typically fairly linear in construction, whether the GM railroads in them to keep the PCs on track or force a particular outcome is a different question. And in sandboxes there can be railroading especially around the edges of the box - unbeatable giants to the north, impassable desert to the south, impassable ocean to the east, unscalable mountains to the west. Or just a bright 'you can't go there' line.
Within the sandbox there would need to be choice of where to go or it's not a sandbox, and having a range of desirability in threat/reward ratios within different sandbox areas does not constitute railroading, I may differ from Hussar there. The traditional megadungeon allows players to go where they want and make their own judgement about likely threat level, how much risk they want to undertake - more risk for likely higher reward - etc.