My biggest hurdle with Meteor so far has been how hard it makes testing and test-driven development. Built-in testing support is pretty much absent from the core framework and has been marked a "post 1.0" feature on the roadmap. If you know your way around Node, you can kind of isolate out most of the global state that the framework uses and write unit tests, but it's pretty ugly (you can refer to my stack overflow question for a sample of the difficulties I've had: http://stackoverflow.com/questions/15191257#15314384)