Good start, but I would ask this...what is your goal behind defoliating at this stage?
If this was my tree, I would be feeding the hell out of it and trying to get branches to pop everywhere I can. So far, it has several main branches, but you really should double the number of branches emerging from the trunk, thicken them, and then
work on ramification.
Defoliating is to either:
A. Build ramification at the tips of developed branches or
B. To give interior buds a shot at some daylight when they would otherwise be shaded out by a massive canopy...push growth back in toward the trunk
This maple doesn't require defoliating to accomplish either at this point. The main branches are still relatively thin and would benefit from a couple years of strong growth (defoliating does the opposite), and there aren't enough branches yet to shade out the smaller, advantageous buds.
No offense, but defoliating probably isn't the technique to apply...yet.
If it was mine (and I'd put it on my bench
), I would be trying to get the branches to grow 18" this year, and in January, I'd wire them, and prune them back to about the profile it has now (Jason outlined this well in RED), using current branches that have doubled in thickness (the placement is already excellent), and some of the new buds that pop as a result of the gained strength from a season of hard growth to help fill it in. Next year, you might need to defoliate...
Look at this documentary on my J. Maple:
http://www.nebaribonsai.com/Nebari_Bonsai_112109/Projects_files/Evolution of Jap Maple.pdf
I'd say yours is about where this one was in 2005-06. (except yours has a better trunk and branch placement)