We've run into an issue where we would like to have a checkbox field defaulted to TRUE instead of FALSE, but the default setting for all boolean Marketo fields is FALSE and this cannot be overwritten without use of a smart campaign (to the effect of Lead is Created ==> Set field to TRUE).
It would be great to have the ability to toggle this at the admin level when creating fields -- it would completely simplify my build. SFDC has this capability, and I'd love to see it in Marketo as welll.