Recently I lost a lot of rep on SO because one of my top answers was migrated to another site. At the time, I thought the question had been deleted.
I was pleasantly surprised (weeks later) when I joined the other Stack Exchange site and found I already had a bunch of rep on it. It made me stop to wonder if any of my other answers had been migrated.
It would be nice if users could be alerted whenever one of their answers gets migrated. Each migration could show up in the user's activity feed, or there could be a page summarizing all their migrated answers.
I would feel much more motivated to participate on other Stack Exchange sites if I knew I already had answered a few questions on it.