What's so special about it? Isn't it just a repository? Or am I missing something? If it's just a repo, Ubuntu has PPAs and everyone and their mother is creating PPAs.
PPAs and the AUR are very different. Where as PPAs contain prebuilt .deb packages, the AUR hosts PkgBuild scripts that typically pull from a git repo and compile a program for you.
I understand the confusion though, because they accomplish the same goal of installing software that is not in the main repos, but in different ways.