It offers a host of new features and performance enhancements, but most exciting is the addition of PhotoMerge, which allows for panoramic and HDR images in Lightroom itself. The images are merged as a RAW (DNG) file and allows for the entire latitude that would be available to the original files. Merging without actually rasterizing the files could be game changing!
Other exciting features include performance improvements through the use of GPU based processing on compatible systems and facial recognition for "automatic" tagging support.
We've already installed the newly released version and will be evaluating it over the next few weeks.