Ability to move breaks & spacers to other measures
Something I've always wanted to be able to do with all breaks/separators that have been placed in a score is the ability to grab and move them to another measure, esp if I initially dragged them to the wrong measure. It seems like a small thing, but to place one incorrectly (which is easy), then delete it and then place a new one seems counterintuitive and a lot of back'n forth between the score and the palette. This would also make tweaking their locations much easier when the initial placements are no longer good and the entire score has to be redone.
Original discussion in http://musescore.org/en/node/21086