Here are some bullet points from the Yahoo! Blog:
– Mojito is a true MVC framework, that?s better! MVC is a battle-tested design pattern, having proven its usefulness in desktop apps, in server-side apps, in, today, rich client-side applications. Those who ignore MVC? often reinvent it.
– But the best part of Mojito is its ability to ?blur? the client/server boundary, to let you write you code that runs on the client, or the server. Or both.