http://vgmdb.net/forums/showthread.php?t=16475
The above rules apply, and generally soundtracks from game/anime/manga franchises will be approved, while for singles it depends. The Asami Imai one you linked is from a game franchise and was reasonably complete.
I don't remember what you tried to submit last year, but if it was a drama single from an artist that doesn't have many (or any) works in the database, and maybe an incomplete entry, that's probably the reason why it was rejected.
If you're not sure if an album should be in the database or not, you can ask before submitting it.