- The mature libraries and deployment environment of the JVM. So your code runs fast, runs almost anywhere, and a lot of work is already done for you.
- Data literals for arrays and maps. One of the shortcomings of Common Lisp. And catches up with (surpasses?) Python/Ruby/Javascript in this regard.
- ISeq interface means that a single set of functions works across all collection types. (Common Lisp has an amazing array of functions for working on lists, but hit and miss for other collection types.)
- Lisp 1. I know this is an aesthetic thing, but I much prefer how the code looks than Lisp 2's.
- Steals a lot of arc's good ideas, like abbreviated syntax for function literals and semantics for arrays/maps in function position.
- Common Lisp style macros. Just more intuitive, for me, than the hygienic type.
- Interesting place in the space of computer programming languages. Dynamic typing, with an emphasis on programming to interfaces, macros, first class functions, immutable data structures, and built in mechanisms to support parallel programming. I don't know of any other language that chooses this set of tradeoffs.