I have mixed feelings about the general direction of this suggestion. Sure dungeon completions should be somehow recognized and why not go with a rather unsubstantial buff to some items? Then again i feel that this would be a rather too direct approach. For me it dulls down the triumphant glory of the first successfull run, since you basically give the message "Here you have an awesome item, but not really". I'd prefer something of more ideational value that doesn't give a player as much of a feeling of missing out until he gets that dungeon completed. Imho that hurts the motivation to play in the long run.
Maybe i'm just tired of how much the "progress bar" (and that essentially would be one) motivation mechanic is grouted in games nowadays.
(Well, i probably just stated the usual concerns about "the grind" and stuff and given your usage of words in the title i assume you are aware of the contra arguments, but i thought they should be written down anyways.)
@glownmelby: Yes it is