Here are some of the differences, but I can't tell if one of them is crucial. I listed three below for which I've no clue what the impact is.
http://clojure.org/lisps
* The read table is currently not accessible to user programs
* Keywords are not Symbols
* Symbols are not storage locations (see Var)
* immutable data and first-class functions
* *first* is clojure's *car* ;)
