Okay, I figured it out.
What I initially wanted was LESS compilation on build, Web Essentials was so kind to do this on save. To deactivate this, in Visual Studio 2012 head over to Tools -> Options -> Web Essentials -> LESS and deactivate "Generate CSS file on save".
According to the Visual Studio Extension page for Web Essentials, minification on build works not by now, but will on future releases. I have to delete the .min.css files by myself and create it again on every build as it seems. :(
Pro-tip for every dev:
If you use any framework, tools, extensions etc. and your new super-powers don't work as expected, don't blame the extension, shut up and do it yourself.