• Safe points: a multicore prerequisite that ensures that ocamlopt-generated code can always be interrupted.
• The best-fit GC allocation policy is the now default policy (and many other GCs improvements).
• Named existential type variables in pattern matching Showable (type a) (x, show : a * (a -> string)).
• Improved error messages for functor application and functor types.
• Let-punning for monadic let: let* x = x in can be shortened to let* x in.
• Module type substitutions SIG with module type T = F(X).S.
• Many other quality of life improvements
• Many bug fixes

See the detailed list of changes.