Other users have answered these pretty well by now, I'd just like to add to the discussion of this question:
>Likewise, I'm confused about how to get around the limitations of mongodb. Say you run a store with a finite inventory, how do you handle concurrent purchases? How about a website to enroll into classes? How about a webforum which can have exactly 5 administrators?
A quick&easy for me has been to just set up a separate REST API to handle critical data like payments and just have a two-server app. Doesn't really add to development time vs a unified server. Meteor doesn't preclude SQL use, it just doesn't support reactive SQL off the shelf like it does Mongo.