The idea is basically that complex web 2.0 design patterns (ajax/comet/memcache/sql optimization/etc.) require a lot of tedious effort to build and maintain, and wouldn't it be cool if you could simply define models, views, and handler functions, and then have the framework take care of everything else.
I honestly think that Luna has the potential to change web development the way Rails did (i.e. by making it an order of magnitude easier), and could transform web apps themselves the way G-mail did (i.e. by emphasizing things like really-real time updating via comet the way G-mail initially introduced the world to Ajax).
I have no idea how Luna is architected, but the concept really resonates with me: javascript-based, web 2.0-abstractions that prevent repetitive things I do in Rails like having to redefine some of the same functions in ruby code and javascript code, and then having to manually keep them in sync.
It's so cool that I think I'm going to try to hack together my own, humble Luna knock-off built on top of Node.js, jQuery, and MongoDB (which uses a javascript query language and JSON-but-better (BSON) data models).