The usual ones, I'm sure. Poor syntax for type lamdbas. Unification across kinds (hopefully possibly going to be backported to 2.11). All the ways of expressing union types (or indeed basic enums) are a bit clunky. Irrefutable patterns in for/yield constructs compile to a silly withFilter call. PartialFunction/Function having the wrong subtyping relationship. mapValues being lazy when all the other functions on the same interface are strict. Stream implementing the collection interface.

More than anything specific though, I don't want to be stuck on an old version as the language moves forward.