Macworld takes a look at Mail in Mountain Lion with their hands-on. There's a few subtle new features such as being able to search for things inline, support for sending new mail notifications, and an entirely new feature called VIPs, but apart from that Mail in Mountain Lion is more evolutionary, and it definitely doesn't contain the revolutionary UI update like it did in Lion. One potential point to note is that it's missing RSS functionality, if that matters to you. Lifehacker has a list of their top 10 downloads for the Mac that extend built-in functionality. It's a good list that contains the like of TotalTerminal, TotalFinder, RestoreMeNot, HyperDock, HyperSpaces, and more — all of which build on existing functionality. If you've been looking for a few new apps to try out without replacing things completely, then have a look at their list.
TUAW shares a Mac app that I've heard good things about, Gemini. It's an app that's will be more useful to some than it will be for others, but if you've ever wanted to find or remove duplicate files on your Mac, then you should definitely take at Gemini.
The Next Web's Matthew Panzarino says Mountain Lion isn't about the "iOS-ification" of the Mac, rather, it's about unification between the two platforms. IOS originally came from OS X beginnings, and even though now it's seen as the more popular brother, there's nothing wrong with unification at all.