I came up with my list without reading any others and came up with variety, variety, variety. Some of these may overlap a bit, but this was my instinctual list...
1) green complex variety (angle to fairway, internal contour, slope direction, hazard locations, uphill/downhill approach, etc.)
2) driving angles
3) hazard variety (mounds, ridges, bunkers, hollows, swales, fescue, water, trees, gorse, etc.)
4) routing, specifically for walkability
5) yardage variety (different length par 3's, 4's and 5's)