Dictation alone is the must have feature in Mountain Lion for me. I'm already loving being able to send emails, make lists, surf the web, etc. without typing.
Notification Center will be nice but right now it's buggy and not all apps work with it.
Haven't spent a lot of time yet on anything else.
I have noticed though that the Display Preferences keep resetting to Best For Display, which is not the native resolution of my monitor, and it's annoying. I'm on a Mac Mini, so I don't have two GPU's like some of the newer Mac laptops. Not sure why this is what it is, but I'm sure it will be fixed shortly.
BTW, if anyone is using Growl, go install Hiss. It forces Growl to use Notification Center.