Awesome, thanks. Edited the first post.
I would like to point at the editor, too. But I fear bloating the sentence. Custom levels don't imply an editor, and an editor doesn't imply lots of readily-packaged levels. Tricky.
What is Lix in my opinion? Singleplayer and multiplayer first. With levels built by the community. Then the editor, to make level creation bearable. Then custom graphics, because I don't use DMA's copyrighted graphics and players have contributed cool graphics. It builds and runs on Windows, Linux, Mac, is open-source, and relies only on open-source libraries and tools.
-- Simon