Letting me talk back and forth with the people in my network quickly and easily about any post they just made. I don't have that now. I often times want to say, thanks for the link, or that's cool but what if, but I can't do that with del.icio.us in its current state.
Allowing the conversation to continue and grow inside your del.icio.us network, now we're talking.
That and changing the damn name to something easier to type.