I think it's three things:

1. Bringing abstractions that are only possible with static types, like ad hoc polymorphism via type classes. For example, type classes allow polymorphism on the return type rather than the argument types. Something like

    (declare stringify (Into :a String => :a -> :a -> String))
    (define (stringify a b)
      (str:concat (into a) (into b)))

    ; COALTON-USER> (coalton (stringify 1 2))
    ; "12"

    (concatenate 'string (coerce a 'string) (coerce b 'string))

2. Making high performance more accessible. It's possible to get very high performance out of Common Lisp, but it usually leads to creating difficult or inextensible abstractions. A lot of very high performance Common Lisp code ends up effectively looking like monomorphic imperative code; it's the most practical way to coax the compiler into producing efficient assembly.

Coalton, though, has an optimizing compiler that does (some amount of) heuristic inlining, representation selection, stack allocation, constant folding, call-site optimization, code motion, etc. Common Lisp often can't do certain optimizations because the language must respect the standard, which allows things to be redefined at run-time, for example. Coalton's delineation of "development" and "release" modes gives the programmer the option to say "I'm done!" and let the compiler rip through the code and optimize it.

3. Type safety, of course, in the spirit of ML/Haskell/etc.