A while back I wrestled Ryan Macklin to the floor and insisted that we talk about some specific game design principles. One of my personal favorites that I keep coming back to is the idea of building "points of tension" into the system itself.
This conversation was recorded as Master Plan #33.
http://masterplanpodcast.net/index.php?post_id=359747 Give it a listen -- it's only about half an hour -- if only to refute my wacky assertion that conflict is underutilized in RPGs. :)