I have thought about this for a while and when you consider the time evolution from putting surfaces built in the 20's and the dramatic styling they have, but still played on today, could some of these and their inherent qualities be considered as 'hazards' as opposed to craftly, clever, interesting features?
This came more to light in rereading Mark Fine's post on exciting shots. If we think about and read many of Mark's and Forrest's enlightenments on hazards and from the many sources quoted in their book and from our own personal experiences with greens like these, isn't it reasonable to summize that for many golfers, a green can also be a hazard? Not necesarilly to be avoided, obviously, but to be seriously dealt with in terms of approach, strategy, its ability to cause severe damage with plenty of potential for 3 and 4 putts!
Many quotes on the Exciting Shot thread discuss "short grass, contour/topography, hazardous conditions that impact your score and anything that can add interest and challenge of a golf hole"
So whether 'informal' or 'formal', doesn't seem to matter as clearly, great contoured putting surfaces, such as those we so often speak of on this site, are indeed hazards IMO
Finally, the quote from Ross that Mark stated really brought it to the surface. "this contouring around a green makes possible an infinite variety in the requirements for short shots that no other form of hazard can call for."
What do you think?