loading bar colors are off, it's hard to see where it's loaded to.
items to drag onto the stage are finite, you should just duplicate the MC/Sprite on click, and add a trash can somewhere to remove objects.
Rotation and "flip" would be good ideas, and would cut down on stage space.
removing the static image at the top is also a good idea, and include all blocks from minecraft (drop-down?)
a larger, scrollable stage would be nice. Put everything in menus instead of just placing it around the stage.
keep going, and use FlashDevelop and AS3 to code your projects. It's not hard once you get used to it (which takes a relatively short amount of time) and saves a lot of time and frustration in the long-run.
The concept is nice, if you make it 3D and (if you can figure out how) and export option, and it'd be something very enjoyable.