I think this is a fun thread to read; seeing everyone's ideas of what constitutes what.
For my nickle, I will say that I think of a quilt as being pieced then sewn or tied together; whole cloth quilts being the exception that makes this rule. I see a comforter as whole or large pieces of fabric, sewn together in very large pieces or one piece each for front and back, with just enough stitching to hold the very puffy padding together.
I also never think of a comforter having binding, only a quilt, but not all quilts. LOL