What's New in Mac OS X Lion - Apple Developer
If you want to become a developer, it's $99/year. But yeah, you get to try this out and do bug reporting and such.
Meh. The only thing that I see on that page that has me excited to move to it from Snow Leopard is:
Does anyone actually give a shit about the apple app store? It's kind of cool that it's a Steam-type delivery system for the Mac, but 99% of the stuff on there is just crap.File Coordination
Coordinating access to files between multiple threads and multiple processes can be difficult and error prone. File Coordination helps eliminate inconsistencies due to overlapping reads and writes by allowing your application to access files and directories in a way that is serialized with respect to other processes' accesses of the same files and directories.