Confluence of angles. If flow lines move in one direction and the greens moves in the opposite, eyes will be drawn. Huntingdon Valley accomplishes this illusion with perfection.
Don't give the golfer a chance to stand on a flat spot to read a putt... golfers standing on tilted ground also have their eyes on tilted ground and may subconsciously over or under read a putt.
Horizon features... if a green and collar are mown tight toward fall off areas, it's more possible to misjudge speed of putts to holes cut close to the edge.
Front to back slope... this just messes with a golfer's idea of convention in architecture. Especially with a backwards teir in the green.
Room over the back, kind of a combination of the last two. I've found that many players tend to really go after putts with room over the back of the green. The 3rd at Beechtree is an example.