Mites

Figured out what to search for, and it turns out that it's a harmless (tell that to my ugly leaves!) mite -- Maple Bladder Galls.

http://www.ext.vt.edu/departments/entomology/factsheets/maplemit.html

Note the end where they say it's "one of the most common inquiries received by diagnosticians." At least it's not just me!

Also, http://www.ppdl.purdue.edu/ppdl/expert/Maple_bladder_galls.html