Great question, @davemc
I don't think there are any hard and fast rules, but I do think there is a delicate balance between scale, quality, and personal touch.
My general view here is that to attack scale you have to create process platforms in which you can have people feel engaged, incentivized, and active without depending on hand-holding. It is just impossible to grow large scale communities with that level of required personal touch, so I find it better to build simple and elegant incentive structures and supporting tools, but then provide a broad supporting function.
For smaller groups, such as your truly dedicated members or MVPs, I think you want the inverse of this - truly white-glove service. It is essential that we take care of these folks, but to not also over reward them.