My second reaction: what happens when the magic ends? When I was new to Rails, I really loved how easy it was to get started with scaffolding, a nice DSL for specifying relations, nifty form helpers. However, the first time I veered a little off the golden path and wanted to do something a little more complicated (that was not supported by the form helpers at that time), I ran into a huge wall. I found out that I actually had no idea how Rails worked. The magic had hidden all complexity from me before, and now the magic failed me and I didn't know where to start doing it on my own. Rails has matured a lot since then (it also dropped some of the magic in favor of more explicit behavior) and my understanding of the framework has grown with it.
Meteor looks even more magic to me. There is so much stuff happening. What if I plan to do a lot of dynamic updates and I want to defer live-updating because I am doing mass-updates? What if I need a hard division between client/server code? What if I want to share the backend with an iOS app or something? Can I use this to build an API? The documentation does not yet fully answer these questions for me.