A slightly surprising one, according to Vincent Donovan (In Christianty Rediscovered) it is this:
The final missionary step as regards any nation or culture, and the most important lesson we will ever teach them – is to leave.
After all the work is done, the pioneer must move on and the gospel must be fully contextualised in the culture that the pioneering work is set. Succession needs to be planned, and local leadership must emerge. According to Goerge Lings (wiring in David Male’s edited book, Pioneers 4 Life):
It is the characteristic of pioneers that they are first in and also first out.
I can think of several notable examples where the removal of missionaries from an area, after they have sown the first seeds and established the first groups of disciples, has resulted in an unprecedented spread of the gospel after the missionaries have left. China from 1950-1990 springs mind. Perhaps God knows what he’s doing!