To be honest, a good to very good green site is one where the slope faces the golfer, perhaps with a little valley many yards out in front. It hardly ever gets old hitting over a small valley to a green on the other side, and if the green is placed mid slope, it gets the things TD mentions -
*a nice backdrop
*a high and low side (if the general slope is also 45 degrees, uphill AND sidehill)
Greens do work better on gentle slopes, as they require less grading to catch up to slope after leveling to 2-3%.
If you add in:
*Trees (only on the west side where they don't block morning sun)
*Other natural features, like
^unique contours that can be taken advantage of, rather than a constant slope
^Rock Outcropping, etc.
*Vistas (like shown above at Pebble)
*A convenient spot for the next tee very close by!
Those are just a bonus, and may take it from a good green site to a great one.