None, except vectors and maps (and sets and regexs and ...) being syntactically first class, and being used heavily, not just in special form syntax but in real use. For a whole lot of programming, maps are where it's at for your data structures.
True, these shouldn't be unfamiliar to a CL programmer (although the pervasive use of maps might well be, don't remember how much association lists and properly lists were used).
As someone who adopted the mostly functional Scheme style I don't find the enforced immutability a bigger difference, but I could well see that being true for CL programmers. And for many/some? the bias against OO vs. CL's CLOS, which many claim is the very best OO system ever (as far as I know the only popular one with a MOP).
I would think that the immutability of these data structures would be a bigger difference, both conceptually and in the difficulty of porting CL code into Clojure.