They've hardened OS X's security model a lot. Gatekeeper + new IPC system (XPC) is great. But the best thing is App Sandbox. They've introduced a lot of new entitlements, like "file bookmarks" that'll make it easy and virtually pain free for most apps to embrace Sandbox. Also, Sandboxed apps can now be "Scriptable" - so users can put scripts in a folder in ~/Library/User App Scripts/com.apple.qt and those scripts will execute "without" any restrictions (i.e. outside of Sandbox). The rationale is that users have intentionally put those scripts in that folder (apps don't have access to that folder), so it's OK to run them.

Application Groups: you (or OmniGroup) can mark multiple apps with the same developer ID, and they can share files, use IPC/XPC directly, a shared file system location for user data, etc.

You can watch session 700 at WWDC 2012[1] that's I (poorly) summarized above.

I think OpenGL on OS X is also upgraded to 4.0 on ML, but I'm not sure. Siracusa's review at Ars (when published later today) will tell us if it's true or not.

[1]: http://developer.apple.com/videos/wwdc/2012/