Thanks for the replies everyone.  Adding a little sugar fixed everything.  I'm not a super taster, in fact, most of the time, I scarf down my food without tasting it at all.  Anyway, when I used to live with a friend, I would always use their olive oil and balsamic vinegar on salads.  It never tasted bitter.  Now, I realized that its because the cheaper balsamic vinegars are just regular vinegars with sugar added.  When I bought more expensive balsamic vinegar (not the authentic stuff, just like 15 bucks for the bottle), there was no sugar in it.  Anyway, this troubleshooting process was helpful.