Suppose you have a WordPress blog that you are using to produce tutorials, and that you would like to split up your posts according to skill level (i.e. beginner, intermediate, advanced) depending on how technical/difficult/advanced the tutorial is. Furthermore, the blog in questions spans several categories, so using these might be inappropriate.
So what's the best way? Using custom field?, tags?, post-type? or even sub-categories? I'm assuming that it shall never be required to retrieve all 'beginner' posts from all categories, though this flexibility would be a bonus.