Probably a big investment, but my immediate thought is I'd love a mobile app. The mobile site was better than nothing, but if you had a slick mobile app, I would use it constantly. So many times I forget little things that I would love to jot down in exactly this type of product.

e: On birthdays, might be nice to be able to add day/month but no year. I'm working through adding some of my contacts into this and noticing small things as I update/add more.