Why did I get this message saying my save data will be reset?! AGH!
First, don't panic. Really. Your save data isn't being reset where it matters -- on our servers. It's just that you play Rock Band Blitz a lot and, more importantly, you own a LOT of content. You own so much content, in fact, that the servers are having a hard time keeping up with how much unique data you have. So all we'll do to your scores is reset what's on your console. Without going into too much detail, this means the servers can kind of shortcut when you play, rather than having to read all of your huge save data each time, which can cause gameplay problems. Your rank on the leaderboards is safe and will continue to reflect your score as your play. Likewise, the data that will show in your Rock Band World profile will be accurate to your gameplay. None of that will disappear.
And no, you don't get the Coin bonuses for playing again. Remember, Rock Central knows you've played those songs already, even if your console doesn't!