In theory, Parallels will also pass along messages generated by Windows 8's new toast notifications technology. However, in testing with Messages, a supposedly toast-enabled Metro app that ships with Windows 8 (not the IM client that ships with OS X Mountain Lion), I was unable to get these to work. Given that Windows 8 isn't shipping yet, and that Parallels has stated that full Windows 8 support isn't yet complete, that failure isn't surprising.
Neither Parallels nor Fusion offer in-app settings to control which notifications you see; you'll have to use OS X's Notifications System Preferences panel to fine tune their behavior.
If you wish, both programs will also integrate Windows apps into OS X's Launchpad. In Parallels, you can add any individual Windows program to the Launchpad by Control-clicking its Dock icon and choosing Add to Launchpad, as you would with any OS X app. Fusion has an option to show Windows programs in Launchpad, but it didn't reliably find all my Windows apps.
Parallels 8 has some other Mountain Lion-specific features, but one in particular caught my eye: It adds a new Open in IE button to Safari's toolbar.
This button comes courtesy of a Safari extension that's installed when you install Windows, though you're not asked about it during that process. If you do a lot of work with cross-platform testing or development of websites, the button is a great timesaver; one click, and you're looking at the current Safari page in IE for Windows. (It's easy enough to disable if you don't want to use it, but I would prefer to be asked before programs modify other programs during installation.)
Parallels also supports some Mountain Lion gestures, including pinch to zoom, swiping between full-screen apps, and two finger scrolling. (Swiping only works when you're in Parallels' Coherence mode (again, when each Windows app is treated just like any other stand-alone Mac app.) In my testing, these gestures generally worked well, though not consistently in all apps. You can't, for instance, pinch out and scroll between tabs in IE, as you can do in Safari.
Unrelated to Mountain Lion, Fusion includes a revamped virtual machine library that lets you organize your virtual OSes into folders, and you can choose between icon and list views; you also get an at-a-glance view of each virtual machine's disk space and snapshots. They've also addressed one of my pet peeves: Snapshots (which record the current state of your virtual system, making it easy to restore or recreate) now get their own window, which means I don't need to stop working just to browse my snapshots. There's a new one-click snapshot feature, too, which makes taking a snapshot as easy as possible. Finally, an embedded help center contains a series of how-to videos that cover many basic Fusion tasks.