Support for Type Signature Sections, allowing you to write (:: ty) as a shorthand for (\x -> x :: ty).
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.
Better performance feedback. The compiler will now attempt to warn you if a requested inlining could not be performed due to missing INLINE pragma
More Backpack is chugging along; we have a new user-facing syntax which allows multiple modules to be defined a single file, and are hoping to release at least the ability to publish multiple "units" in a single Cabal file.
A (possible) overhaul of GHC's build system to use Shake instead of Make.
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.