Unless I'm talking about Erlang, my answer is almost going to be "I really miss Erlang's pattern matching". (I know it's not exclusive to Erlang, but that's where I learned it).

My dream language right now would be something with the expressiveness of Ruby and the concurrency abilities and pattern matching from Erlang.

Hmm. I think I need to go look at where Reia (http://reia-lang.org/) has got to since I last looked at it.