Stoneyhill has steep slopes, close-by O.B., trees inside the circle, or elevated baskets on almost every hole---often in exciting combinations. The one safe green really throws off my game.
One other factor sometimes is available is an open hilltop green in places subject to a lot of wind.
I think you can separate challenging greens into various categories:
---Harder to make the putt. Most often trees inside the circle, but also elevated baskets and perhaps tricky footing.
---Consequences of missed putts. Steep slopes, where even a putt that falls out of the chains may roll away.
---Risk/Reward, with a pucker factor in deciding whether to run at the basket or lay up. O.B. behind the basket, big downhill putts, etc.
A variety of these situations on a given course can be fun. Too much of any particular feature, not so much. Think before putting all baskets on slopes, or all near O.B., or all different heights, or all with lots of trees on the green.