ELPA's pretty nice, I've only got two minor issues with it:
* It doesn't have everything in it yet, so about half the packages I use I still have to manage manually. (Someday I'll have to figure out how to submit stuff to ELPA...)
Clojure runs on the Java virtual machine, so you can run (mostly) anywhere and take advantage of the huge amount of libraries. It also has built-in support for concurrency with software transactional memory.
Also, its common data types (lists, vectors, maps, sets) are immutable and use pointer sharing to make changes extremely cheap. This makes reads always safe and lock free, but performance guarantees are slightly worse- O(log32N) for vectors instead of O(1)- but still good enough that you probably won't care.
To me that's just as impressive and important as the STM, but I rarely see it mentioned.
To me the biggest advantage (which Arc shares) is that the core language can change if it is advantageous to do so.
Scheme has the RnRx process, which seems slow moving to me, and lately steeped in a little more drama than I like. I'm not sure when the last time the ANSI spec for Common Lisp was changed.
I know you can change Common Lisp and Scheme via libraries, but that's not the same as everyone having the same functionality when they are on the same versions.
