For me, some of the important things are:
* The language is much smaller and cleaner than Common Lisp (and designed by some of the same people that participated in the CL standardization).
* Because it has a smaller community and installed base at this point in time, it has the freedom to evolve in new directions whereas an existing Lisp is already defined and tied down to what it is (for the most part). (Coroutines would be a nice addition for example.)
* You can produce executables from it and don't have to have a running image.
* The integration of the type system is, for the most part, nice. It is nice to have the ability to have a mix of dynamic and more static typing. (The type system could be better, and that too is an interesting possible future direction.)
* Always using multi-methods and everything being an object and the more powerful dispatch capabilities are quite handy.
* It has a condition system like Common Lisp rather than exceptions.
* The macro system is okay.
* It is nice having multiple return values, #rest parameters and keyword parameters, all cleanly integrated.
* The C-FFI is good.
* Some people like the syntax. I enjoy how readable the code is to me.
* I'm not tied into using the JVM.
Some Lisps have some of the above, some don't. Not everyone might like all of the above, but for me, it is a comfortable fit.