There's really not much Lisp/Scheme left in R these days.

Function arguments/environments are still pairlists: a type of cons cell.

You can manipulate the parameters to a function as symbols before you evaluate them. This is useful — many very good R libraries all make heavy use of it — and certainly very interesting. Lispers probably know it as quote/antiquote.

I suppose we could also point that, much like most Lisps, R gives you plenty of object systems to choose from...

And that's about it for R-as-a-Lisp.

It's also a really bad language for working with tree-structured data, something Lisps normally excel at.