Support for Type Signature Sections, allowing you to write (:: ty) as a shorthand for (\x -> x :: ty).
A huge improvement to pattern matching (including much better coverage of GADTs), based on the work of Simon PJ and Georgios Karachalias. For more details, see their paper.
A (possible) overhaul of GHC's build system to use Shake instead of Make.
Support for reasoning about kind equalities, which gives promotion of GADTs to kinds, kind families, heterogeneous equality (kind-indexed GADTs), and * :: *. There is some discussion in DependentHaskell/Phase1, but that's very low-level. I (Richard) have no good user-oriented write-up yet, but there shouldn't be much in the way of new syntax -- just fewer type errors.
A new, type-indexed type representation, data TTypeRep (a :: k). This change should be fully backward compatible. See Typeable.