Rule 1: Good Design is Transparent.

So here's a shocker.  You shouldn't notice good design.  Design is usually only apparent to us when it gets in the way or fails to accomplish a task we desire of it.  This happens all the time, from real life to virtual games.  Whether you're cognizant of the design issue, or just annoyed by a particular issue, you've been affected by bad design.  Using a spatula and need to set it down and looking for a clean place?  That's a design issue.  (Of course, that underscores a future design rule.)

There is a corollary to the first rule though.

Corollary: Good Design is Transparent, Unless it is INTENTIONALLY designed to get in the way.

Case in point:  In the bathroom stalls at some airports, the shelf for holding one's bag is placed in such a way that it blocks the door when used.  It's large, obtrusive, and just a pain in the ass.  And that's the point.  It's so in the way that there's no way to forget about your bag after using the bathroom.  Seriously, it's huge.  I'd post a picture of one, but Google Image Search has completely failed me.  I guess people take pictures of their toilets before they take pictures of their stall doors.