Now that ruby-libs has been split out of ruby, the explicit dependencies should go away. There are also a number of annoying dependencies such as mysql, jack, etc. Isn't it possible to drop them somehow? (probably requires work upstream)
The dependence on ruby is there because of the lyrics script (which is run by default) The dependency on MySQL is there because Amarok can store its music collection in MySQL (and PostgreSQL). To my knowledge there is no way to make it a plugin at the moment. Someone already posted an RFE upstream : http://bugs.kde.org/show_bug.cgi?id=102318