• A new intermediate representation, called Flambda, was added to the native-code compiler, along with several new optimizations over the Flambda representation, such as aggressive function inlining. (The Flambda optimizer needs to be activated at configuration time by ./configure -flambda.)
• New language feature: inline records as arguments to datatype constructors. This makes it possible to name the arguments of a constructor, and use convenient record notation to access them, without the cost of declaring a separate record type for the arguments.
• The @unboxed and @untagged attributes are supported on external function declarations to pass parameters and results to C stub functions in a more efficient way. Other attributes honored by the compiler include @tailcall and @inline.
• Improvements to the garbage collector, resulting in better GC latency (shorter GC pauses).
• Support for ephemerons, a more general form of GC finalization of data structures.
• The runtime system is now compiled at higher levels of C optimization, resulting in significant speedups for the bytecode interpreter.
• New native code generators supporting the PowerPC 64-bit architecture (in big and little-endian modes) and the IBM zSystem architecture.
• The whole code base (compilers, libraries and tools) is now licensed under the LGPL v2.1 with static linking exception.
• The ocamlbuild compilation manager was split off and lives as an independent project.

See also: detailed list of changes.