I'm surprised no one has mentioned: sugar
if there's a single place I disagree with Alton Brown, it is his statement that thou shalt never use a pancake recipe for waffles.
actually, I find a pancake recipe, slightly adjust for consistency, to make marvelous waffles.
all that said, sugar aids in the crisp factor.
other things I've learned as crispy no-nos:
no flat cooling surfaces - use a rack
no cover anything - covering a waffle to keep it warm does not make = crispy
I hold deep pocket waffles in a 250'F oven on a regular basis. got sugar, got crisp, nadda problem holding them.
the leftovers get froze for toaster waffles - and thems be crispy, too.