The premise of this explanatory and insightful commentary on African religion is that a Kenyan minister is hoping to provide a more holistic, less simplified view of African religion, hopefully one that helps to adjust the terrible bias that many Westerners feel against the continent. It does this by returning to the truths that African culture celebrates, like the importance of accepting one's responsibility and duty, understanding one's self as a member of a community instead of purely and individual, and a deep reverence for life and the earth. These virtues are shared by Christianity, so instead of seeing African tribal religion as an offense against God, Mbiti hopes to show the strange practices as demonstrations that African people know God deeply and love and respect creation.
One important philosophical difference that helps to make this more clear is Mbiti's discussion of time in tribal communities. Instead of regarding time as a commodity, they seem oblivious to time entirely, except for cyclical time which they understand very deeply. Without this sense of time as a resource that can be wasted, Africans are more likely to take pride in the major events and accomplishments of their life, instead of being bogged down by their daily performance. This peacefulness is deepened by the natural instinctual quality of their life style. Because they are close to nature, they are close to God, at least in Mbiti's experience.
The idea that humans continue on as ghosts is common in African religions, but that doesn't imply anything about the afterlife. They speculate about spirits and gods, but to concern themselves with the moral question of the afterlife isn't necessary it seems for a fully developed religion.
That's another important thing to say on Mbiti's behalf—his argument against the evolutionary idea of religion is seriously formidable, and the comparison between the monotheistic attitude and behavior of African tribesmen and Christians is also not something to be taken lightly. To reduce African religion to an evil society with witchcraft and the dark arts would be to miss the beauty of their perspective, and in many of the essential ways, Africans share ethical and moral opinions with Christians. This is Mbiti's analysis, anyway.
The truth is that in order to understand African religion entirely would be an impossible task, because of the size of the continent and the variety of tribal systems and beliefs. However, since the primary goal was to provide a new lens for interpreting African tribal life without condemning it, the work is successful. It's not an amalgam of every religion and philosophy, but rather a correction of the Western bias against African tribal cultures.