* Tagged template strings might have some potential to be abused in interesting ways, since it looks like the tag function doesn't actually have to return a string.
* The Object destructuring/enhanced literals (and the Object rest/spread proposal in ES7) still take a lot of mental energy for me to parse. I'm worried they could be a maintenance liability in large projects, but that remains to be seen.
* Async functions/generators in ES7 will probably take a while to get used to, as well, but I felt the same way about Promises when they were first introduced, and they didn't turn out to be too bad. I don't have an issue with normal Promises/generators, so this might just be a matter of me needing to see the async stuff as a whole instead of the sum of its parts.