Meteor is not just a web framework which many actually want. I can't just build a new frontend for my existing Django application with Meteor can I? Meteor is server & client with also a lock on Mongodb.
Of course not. Meteor is not a frontend framework that will fit yor X backend. Meteor is an Isomorphic Environment that, by the way, rocks if you have the freedom to choose it. Completely different things.
Regarding to MongoDB, the Postgres option is in the pipeline.
Regarding to MongoDB, the Postgres option is in the pipeline.