No definite accession principle. Old Arab tribal customs were retained just with an addition of hereditary.
Ill-treatment of Generals and Governors.
Arab Tribes rivalry. The jump from tribalism to feudalism proved fatal. They preserved the kinship society but introduced private property ownership.
Himayarites versus Modharites.
Irking Arab settlers.
Engaging in harem practices.
The power of Abbasids themselves.
For complete Islamic history notes click here.