I played a course today which is grazed, so great use of the land in terms of providing food for my belly and keeping rough managed. The fairways were fairly narrow, but they didn't need to be wide with managabe rough in nearly all places. There are a few opportunities to lose a ball in hazards, but not many. Basically no fairway sand, so again, quite managable. No par 5s so yardage less then 6000 from daily tee. Courses such as this are what I would call playable. There are other more testing designs which I would say are playable, but just as Adam states, its about a balance. Its not a perfect formula for playable, but once we go down the road of formulas then we may as well stop searching for good golfing country. Everytime test, challenge or championship design is mentioned its one more step away from playable, but that ain't necessarily bad. All courses can't be all things to all people....thankfully.
Ciao