What are the essential elements of Wikipedia's success?

Wikipedia has been wildly successful. If we wish to see the continuation and replication of this success, we will need to understand what are the essential factors that have made success possible. Otherwise we risk changing the things that are most important.

If we were to ask around, "Name the thing most responsible for Wikipedia's success," I suspect we would get a broad range of answers. I especially suspect that people would tend to single out those factors that support their ideological presumptions: "I think 'x' is good, so I will believe 'x' is the essential factor for Wikipedia's success; then I will take the success of Wikipedia as proof that 'x' is correct." This doesn't get us very far.

If we are mistaken about what is the most essential factor for Wikipedia's success, we will waste a lot of effort and stifle innovation trying to preserve a thing that doesn't really matter.

There may be many factors that are critical, such that the essential thing is not one of these factors, but the relationship or interaction of them all. Or not.

That said, I would suggest the following as the/an essential factor, and would hope to see other suggestions from other people:

Costs per unit of quality increase on a linear, rather than exponential, scale:

In most systems, a linear increase in quality is accompanied by an exponential increase in costs. Each unit of additional quality costs more than the previous. Before quality approaches perfection, costs increase to prohibitive levels.

Traditional print encyclopedias are effected by this phenomenon. Quality can be increased only on a linear scale, while the costs per unit of quality increase on an exponential scale. At some point, the cost of additional quality becomes too high to be worthwhile. The precise location of this point may change with changes in technology and economic factors, but the point will always exist.

Wikipedia is unusual in that the costs per unit of quality increase on a linear scale, rather than on an exponential scale. Doubling the quality can be achieved with a mere doubling of costs, more or less. Quality can increase indefinitely without costs spiralling out of control.

If costs per unit of quality increase only on a linear scale, the point where the cost of additional quality becomes too high to be worthwhile will never be reached, and Wikipedia's superiority as a reference source obtains a certain inevitability. This may be the most powerful fact about Wikipedia, and the essential, most necessary ingredient to its success.