I think the 6 is generally given for playability and effort. I imagine you'd have to play a game that's fundamentally broken, buggy beyond control/progress, or somehow clearly unworked on. Yes, the reviewer's scale has been chopped down to 6-10, blah blah, but most of these reviewers, cannot in their right minds, give a product 1 or 2 when dozens of people have spent hundreds of hours working on it. Remember that despite repeated cries to the contrary, most people hold reviews to be more of an objective evaluation than an opinion.
For example, think an RPG with no textures, where the NPCs are just moving black blocks, controls become unresponsive every few seconds, the game itself is unplayable on most systems, events fail to trigger and the game just doesn't work at all, and is still marketed as a finished product. But in such a case I'd probably just throw a consumer advisory rather than bother writing a review.