I suppose there are a few things that you can put on there, though they aren't any good metrics (unlike for example horse power with cars which give you at least some kind of information):
- Size of the standard library (which has the downside of languages which provide a huge and extensive library themselves are getting bigger numbers, while languages merely providing a wrapper over other things get lower numbers, without providing less functionality)
- # external dependencies (that's highly platform depended and a bogus number anyways)
- Time to execute xyz (which requires you to write a test program which is fair for all languages. Good luck with that, Erlang for example is useless for most tasks, but shines when you try to write a Jabber server or a telephony system)
- Number of page in the language spec (which C++11 with over 1100 pages would win by far, though not every language is standardized and provides a spec, PHP or Perl for example)
- Number of lines to express the language Yacc grammar (not sure if you can find one for every language. C++ is going to be hard, because the standard deliberately doesn't provide one for a few things since they can't be expressed in any way)

Stuff like that... But yeah, you see the problem with that I guess.