PouchDB also has quite a few useful plugins (full-text search, geospatial queries, peer-to-peer WebRTC replication, etc), runs on the server, and is LevelUP compatible.
Primarily I've been focussed on getting the browser side solid and reliable in production, sync is definitely on the way though. Don't forget that PouchDB is designed to work with CouchDB and not MongoDB so Mongo lovers are somewhat in the cold right now with client-side counterparts.
For MongoDB lovers, PouchDB supports MongoDB-style querying via https://github.com/nolanlawson/pouchdb-find. MongoDB doesn't have the fundamental sync primitives that CouchDB has so I wouldn't expect a similar solution which supports MongoDB on the server.