In what ways have the dwarves let Bilbo down? How does Bilbo differ from them?

The Dwarves never really believed Bilbo. Bilbo's motivations for going on the quest for Erebor were not the same as the Dwarves. The Dwarves wanted to reclaim an old ancestral home and avenge their kin while Bilbo went because he got angry when he heard the dwarves speaking disparagingly of him. Bilbo knew that they underestimated him and was determined show them what he was worth.