I'm like hefdaddy, I try to keep my gaps to a minimum.
Something I haven't done alot of though would be english literature and/or arts & culture. I've done analytical english (things like critically reading articles, creative writing etc) but never anything with a whole lot of depth. Its something that, looking back now, I should have done more of so I can appreciate the arts better.
I suppose if we are going to be broad, I'm ignorant when it comes to musical instrumentation also - which sucks when you love music so much.