Rust 0.8 development was mostly focused around refining Rust's standard library. Of the language changes to the release is making the "for" keyword work with Iterator types, a re-written run-time and task scheduler, a new experimental I/O subsystem, and a new family of string formatting macros. There's roughly 2,200 changes to Rust 0.8 with many of them being bug-fixes and minor enhancements.
For those unfamiliar with Rust, it started out as a Mozilla research project as a new safe, concurrent, practical language that's compiled using LLVM as it's back-end. Rust has been in an alpha state since early 2012 and still has huge aspirations within the Mozilla world.
More details on Rust 0.8 can be found from its release announcement and its supporting links for downloads and the release notes in full.