Most of the Go 1.1 improvements involve performance, including optimizations made to their compiler, linker, garbage collector, goroutine scheduler, map implementation, and its standard library. Google says that existing 1.0 applications are likely to be much faster when simply built by the new Go 1.1 stack.
Go 1.1 also has changes to return requirements and method values, safer concurrent programming, and the tools and standard library has been improved upon. Go 1.1 remains compatible with Go 1.0.
More details on Go 1.1 can be found from the blog announcement and release notes.