CoffeeScript hides the most hideous warts of JavaScript. It's worth it because although messy as Hell, JS has an amazing ecosystem, most importantly/obvioulsy the ability to run in a client's browser.

If I get it right, MoonScript aims at doing the same for Lua. The difference is that (1) Lua ecosystem isn't nearly as unavoidable as JS' and (2) Although some design decisions in Lua aren't consensual, the one thing you cannot call the language is "messy". IMO, Lua's main drawback is that by being so flexible and lacking an interventionist benevolent dictator, it doesn't grow standard coding practices, as Python does. Of course, creating alternative syntaxes isn't likely to improve the matter.

But in the end, it's fun to write compilers, and this is a perfectly legitimate reason to write one.

The lua ecosystem is extremely simple to embed in a native application, which, as someone who has had to do it, is something that feels underappreciated. Python and javascript were harder to set up for embedding on the same computer, and all but impossible to cross-compile for iOS. The simplicity of using lua as a scripting language in larger applications makes it a great target for compilation.

MoonScript also gets rid of a lot of the more verbose parts of lua, and replaces them with CoffeeScript-like versions. For example: shorter anonymous function syntax, syntactically significant whitespace, default local variables.

You might as well ask what is so great about Lua. This kind of thin compiler is what you get when you want the semantics and the ecosystem of Lua but with a richer syntax.