An Imperial Roman bureaucracy which hadn't changed since Diocletian died.
The Emperor had complete power over the state because of that and only had to contend with the Senate and army.
The underlying foundations of the Empire were extremely stable it quite literally took the fourth crusade to dismantle them. While in the East and West states were divided and could be further there was only one possible state in the Empire and that was their Empire.
They kept most of the Roman military traditions from the Late Empire and even kept the Legions in the form of the Tagma in just about the same military role and same military units from Constantine.
The Roman economy was completely monetized which allowed the Emperors to collect tax in coin rather then barter which could be used for military and administrative expenditure far easier then a cow could.
It was only until the 11th century where Western Europe caught up militarily as professional and disciplined armies were no longer the sole privilege of the Roman state. Still than they held many advantages which were to be envied.