Nothing major has ever been erased of mine, because I am very specific when I say what not to delete.
However, my sister claimed our family Wii as "hers" when she moved out, so I lost a lot of stuff. For a small list...
- All unlockable characters, songs, stages, and game modes in Brawl
- All unlockable characters, karts, bikes, and tracks in Mario Kart Wii
- My entire 100% TP file
- All unlockable characters, guitars, finishes, songs, outfits, etc. in Guitar Hero 3, as well as all but 2 songs completed on Expert Difficulty
- All unlockable songs in Rock Band, as well as completion of all songs on all difficulties for Guitar and Drums, as well as all songs completed up to medium difficulty for Vocals
- My nearly 100% Super Mario Galaxy File
- All of my VC games
- A crapload of other data that I didn't care much for
I happen to have another Wii in my household, so most of that data has since been recovered. Sadly, though, I still have a long way to go in some of those games. I actually haven't even played that Wii since she left...
But ya, I was kind of ticked at first...