"The biggest problem in Kenya - and across sub-Saharan Africa - is not poverty but inequality. Many African countries are growing at rates of 7%-8% a year, but this is destabilising if it is not accompanied by equity. In highly heterogeneous societies, structural inequality is easier to politicise, and you do that by ethnicising it - as happened in Kenya in 2008. And then you militarise these conflicts using party youth militias. That combination fundamentally undermines democracy because it leads to mobilisation along ethnic lines, and that becomes toxic. You can blame an entire group for your woes..."
John Githongo, the Kenyan anti-corruption campaigner who once served in the Mwai Kibaki administration and was exiled to London for speaking out. I am not sure it is the biggest problem, but it is up there with other huge problems like absence of rule of law. At this point it helps to distinguish, as we always admonish, between "proximate" and "ultimate" causes. Inequality is clearly a proximate cause. For further discussion of the problem of inequality see here, here and here.