If you mean changes between 0.7.15+ and 0.8.0, nothing really significant. But if you mean changes between 0.6.x and 0.8.0, well, almost everything has changed!

Sorry for the non-answer... But take a look at https://github.com/joyent/node/wiki/API-changes-between-v0.6... and https://groups.google.com/d/forum/nodejs-dev for change log (in 0.7 branch).