Bad DMing

[atemu1234]

Other than the things mentioned in the DM's Guidebook what are other bad things a DM could do?

[Kazudo]

Railroad mercilessly.

The way I've seen it, open-ended sandbox type games rarely work without the right group. A lot of gamers require plot. However, forcing characters down your plot without letting them progress it at their own rate in their own way is just blatantly unnecessary, and if the group decides that the adventure they were on isn't fun anymore, they should be able to have the option to have another plot.

With that being said, (granted, this is just my ideal setting, I'm only one person), the best DM that I ever had had a large overarching plot written in broad strokes, and let us fill in the details (sometimes in ways he didn't foresee).

Another thing would be penalizing cleverness.

I'm not talking about killing catgirls and whining when you're being told you're "breaking the game" (which I believe is a myth, personally, but to each his own). I'm talking about someone finding a unique answer to a situation that should work or has worked in the past and is being directly penalized (more than simply being advised that due to different circumstances or things outside of the character's control, it didn't work for some reason).

Basically, don't make the game unfun.