Bad design, whether it's railroading, DMPCs, or boring settings, all have one thing in common: They put inert elements in front of players (a.k.a. they're all a waste of time).
"Here's the prince of Roundheria, he's thinking of invading the peaceful land of Overtheria, but you can't convince him not to."
"Here's the dragon of the west mountain, but he's so powerful he'll kill you all instantly, so listen to his monologue before he flies off to what I've already decided he's doing, OK?"
"You're in a town. There's an inn. No, nothing interesting is happening."
"Elminster talks to you for half an hour, here's his 20-page explanation of why he's not going to help you. No, you can't convince him."
In short: if it comes time for the DM to add some element to the game, and it's not a problem, threat, resource, or reward, it's a waste.