On average, I send about three emails a day to people I may have never emailed before, or have but don't see regularly. In both cases I forget that I've emailed them, so if they don't reply then the conversation is 'lost' forever.
My ideal solution: I know Gmail stores the message, but I want some kind of added notification system, or dashboard, or report that says "Hey, in the last 5 days you sent out 20 emails that still haven't been replied to yet." And I can see which ones still haven't been replied to and then re-send a message.