From Is RethinkDB the next DB singing on Meteor? - #2 by mquandalle
"…what makes the integration of any non-MongoDB database with meteor especially difficult lies in the 3rd principle of the platform:
Database Everywhere. You can use the same methods to access your database from the client or the server.
Which basically means that we have to re-implement a full database on the client (minimongo, miniredis, minirethink, etc.), which is non-trivial…"
“…the tricky part is the wire protocol (livedata) and the client “mini” database.”
Note that Meteor uses oplog for improved reactivity speed but can fall back to long polling (every 10 seconds).
Personally I’m hoping RethinkDB will get some love either in core or as a package.