As a Java developer I would need a few things to switch and maintain my current level of productivity: generics, a map/filter/reduce paradigm for working on collections, and more flexible error handling -- in that order.
Oh, and let me leave "fmt" in the list of imports even when it's unused, because adding it every time I stick in a printf to debug something, and then needing to remove it later because of compiler errors, is incredibly frustrating.