Leveldb is an awesome key-value store, but it is not really a database. So if you need anything like relationships, queries, transactions, secondary indexed or any of the other features that defines a database, you are pretty much left out in the cold.
Totally, don't get me wrong - I think this is very exciting, and love me some relations. It's just that given all the other data stores mentioned on the page, leveldb is conspicuous in its absence.
http://kellabyte.com/2013/06/06/leveldb-was-designed-for-mob...