Unboxed vectors, Boxed vectors, Packed UTF8 String, Bitmap.
Polymorphic API working on many types similarly; let the types drive the implementation detail
No more hard-to-fit Num class, replaced by Additive, Subtractive, Multiplicative ...
Apart from Haskell base (and its core libraries like ghc-prim)
A common place to fix bug and add feature
Simpler versioning for everyone, less independent bits to track