Here are some of the "gotchas" I've encountered when I converted all the javascript code into coffeescript. (BTW, this is gathered while coding for YourMechanic.com, a very cool mobile auto repair service.)
Disclaimer: Coffeescript is a great productivity booster for me personally. Even with all the things outlined below, it is still way better than javascript. None of these should discourage you from jumping into it right away.
(Things that generate wrong code silently)