The update cleans up inconsistencies over platforms, says Daring Fireball's John Gruber:
The recurring theme: Apple is fighting against cruft — inconsistencies and oddities that have accumulated over the years, which made sense at one point but no longer — like managing to-dos in iCal (because CalDAV was being used to sync them to a server) or notes in Mail (because IMAP was the syncing back-end). The changes and additions in Mountain Lion are in a consistent vein: making things simpler and more obvious, closer to how things should be rather than simply how they always have been.
Apple has updated iChat to appease iMessage users, according to Siegler. "It technically replaces iChat, but with some tweaks, you can find that old interface as well. But the focus now is on a unified dashboard of all your messages, meaning yes, iMessages as well as IM messages. Heavy iMessage users are going to love this."
That means integration, continues Dalrymple. "Here’s the great thing about Messages. It keeps the conversations synced between devices."
But that also can lead to problems, adds Patel:
The first problem is simply structural: not everyone uses iMessage for everything, and it’s easy to find yourself looking at an unwieldy mix of iMessage, Google Talk, and AIM messages all from the same person at the same time. People use different message services for different things, but Messages wants to mix everything together in a way that doesn’t always make sense — and only iMessages will sync across devices, so you might find yourself missing pieces on iOS devices. iMessages sent from iPhones that fall back to SMS also won’t show up, which can be confusing.
A new take on Mac notifications, writes Patel. "Mac users have relied on the excellent open-source Growl notification system for years now, so the addition of Notifications Center to Mountain Lion seems a bit anticlimactic at first glance. But look a little deeper and it's clear that Apple has a very different idea about how notifications should work — an idea that's unsurprisingly almost exactly how they work in iOS 5."
Slicker than Growl, adds MacWorld's Jason Snell. "Apple’s take on the concept is slicker than Growl, though, and if it ends up being supported by more programs, it’ll be an even bigger boon."
Even better than Lion, says Siegler. "With Lion, iCloud had a number of points of integration. But they were tacked on after the initial release. With Mountain Lion, the connection is much deeper. Actually, from the first screen in the setup assistant, you’ll now be asked to set up iCloud."
The whole connection process is seamless, from Dalrymple "The iCloud integration is superb. It’s not just registering your computer, Apple made it so iCloud sets up many of the applications and services on your system for you ... For example, iCloud in Mountain Lion works with Mail, Contacts, Calendars, Messages, FaceTime, Notes, Reminders, Game Center, Mac App Store, Documents and Data, and Bookmarks."