[dsyko]

To answer your first question, there are a number of attractive advantages to using Meteor over say socket.io or autobahn on top of Node. One major one would be a consistent API for accessing your data on the Client and Server. I would argue that using Javascript on both backend and frontend with Node boosts a developer's productivity by reducing language/context switching. In the same vein accessing data using the Mongo query api on both backend and front end with Meteor's MiniMongo improves development. Also Meteor comes with built-in latency compensation when using either RPCs (with stubs on the client) or Minimongo operations. Latency compensation is a non-trivial problem which would need to be implemented by the developer when using something like socket.io. You also gain access to a growing list of great packages that implement things like user accounts which you would otherwise need to write yourself or wire together from various projects.

As for your second question. There are certainty use cases where MongoDB isn't appropriate, and support for other databases is on Meteor's roadmap. For now there are some community created packages for interfacing directly with SQL databases in place of MongoDB, but there are a number of issues with them. You could also pipe data in and out of an SQL database inside RPCs, however the data would not be updated in 'real-time' to clients. I anticipate SQL databases will get some good support options in time.