Haxe is an open source toolkit based on a modern, high level, strictly typed programming language, a cross-compiler, a complete cross-platform standard library and ways to access each platform's native capabilities.

I have been looking for an excuse to use Haxe but the lack of documentation and tutorials are really hampering my adoption, there are a lot of tutorials however for game oriented projects, not for anything else.

What is the killer feature of Haxe?

For me personally, it's being able to focus on one language, and one standard library, and be able to re-use that skill-set everywhere. Whether I'm working on a server-side web app, a client side JS framework, a CLI tool or a mobile game, I can use the same language, same libraries, same idioms, but a completely different target platform.

In my work I've targeted JS (browser), Neko (server), JS (server), JS (node-webkit), C++, Java and Python - all using the same language / standard library. It means my brain doesn't have to switch gears as often when I need to make something to run on a different platform / technology. Plus there is a huge amount of code re-use you can achieve across platforms.

You can get something somewhat similar with JS thanks to Node (targeting client + server + CLI), but I find Haxe a much nicer language to work with - both for Object Oriented style code and more functional style code. The GADTs, "Abstract types", and meta programming (macros) are all features I think Haxe implements in a more well thought out way than other compile-to-JS languages.

In my opinion there is none, it was focused on flash developers, IMO now we have tools much better like Unity3d and UE, b/c I see haxe as a game development tool, don't know others usages right now of haxe

Re-use the same code in a client and on a server is pretty swell. Also being able to target both native (desktop/mobile) as well as browser (js/flash) from the same codebase is great.