In Freenode's #mercurial, sjl said this about common lisp:
<sjl_> CL code from the early 90's runs just fine on SBCL from a month ago
<sjl_> But trying to run five-year-old python/ruby/scala makes me hate life.
This echoes his earlier blog post, volatile software:
http://stevelosh.com/blog/2012/04/volatile-software/
I kind of have the feeling that fighting bitrot is sjl's main motivation for CL.