I really like the the simplicity of the plugin system. Adding (Ka)TeX support, for example, was just a matter of few clicks. Creating plugins looks straight-forward too; you can extend the site or filter / decorate text. This makes me want to use GitBook. If they don't support an idea I know I'd be able to write a plugin for it. And with time and their steady growth I probably wouldn't even have to. Sure, it's very new and has bugs, but I think this has a lot of potential.