Apple previewed the next major update for OS X called Snow Leopoard at the WWDC 08. Apple said that the Snow Leopard release will enhance the performance of OS X, and improve quality and not focus on new features.

Of course there are new features to expect. Apple Insider has just published a report that describes 5 features that have not been disclosed yet.
The new rumored Snow Leopard features are new multi-touch framework, file size reductions, text-processing features, auto activation of fonts, and full ZFS support.

There are already other features known that will be in Snow Leopard like SproutCore, the LLVM Compiler, and native exchange support in Mail.
More details on Apple Insider.