"Under the hood"? Not really. There are a few nifty new APIs – SceneKit, NSXPCConnection, NSUserNotification, the sharing stuff – and the 64-bit kernel (with kernel ASLR) is now the default. Regarding user-visible features, I think there are a couple nice ones:

- AirPlay mirroring is slick, especially in meeting situations

- Document auto-save and versioning are a lot less annoying and more useful

- Improved sharing support can be quite useful in many situations

- Calendar and Contacts suck less

- Gatekeeper is, I honestly believe, a solid step forward in improving system integrity and security

- iCloud sucks less and is better integrated into some system apps

- You are now notified of both system and app updates, and update checks happen daily. I think this is huge for app developers

- Preview has support for entering text in text fields of PDF documents which aren't true PDF forms (this honestly might be my single favorite Mountain Lion feature)

For $20, I think this is a solid "polish" upgrade. There's nothing mind blowing, but there's an incredible amount of polish across the system, with a few truly useful features mixed in. For me? Money well spent.