Jason@Wasted Lands implies that the classic dungeon can be seen as a railroad, because it is pre-scripted and offers limited choices. I disagree. Here's how to make it one, though.
"You have to pull the lever, or there's no more adventure. Trust me."
"You can't turn back to the town. You haven't cleared enough rooms this session yet. OK, if you insist - A Mysterious Force Blocks You."
"Your Hold Person mysteriously fails - that 1 I rolled is actually a 16. The sorcerer drinks a potion, cackles 'I'll see you on Level 6', turns gaseous and disappears down the grate."
(reading mind-molesting boxed text) "Seeing the skeletal lovers' embrace, you cannot help but sigh, shed a tear, blow your nose, and think of lost loves of your past. Then the lovers stir and turn their bony skulls towards you, and you scream in bowel-loosening terror!"
See the point? The issue is not the linearity of an adventure (though branching adventures are more fun, it's true) but about DM vs. player input into the action. Within the confines of even a linear dungeon, the players control the action, tactics, and pace. It's the intruding hand of the DM, not the passive confines of dungeon walls, that grows irksome.
At the same time, I join with Jason in toasting the story DM who stays on his or her own side of the net - setting up NPCs and organizations with their own unfolding stories and agendas, but letting the PCs interact with them as they please, and enjoying the surprises that grow from the mutual interaction.
More passive dungeon DM vs. more active story DM? It's really a matter of taste, preparation, and comfort level with winging it. It's not uncommon for a DM to start a campaign as a fairly passive adventure referee, and grow step by step into an interactive story teller, as little details of the world outside Adventureland grow and build on each other.