A major factor in the success of Orthodox conversion among the Slavic peoples of Eastern Europe was the splendor of Constantinople itself. Numerous accounts survive of the sheer impact Constantinople’s size, prosperity, and beauty had on visitors. Constantinople was simply the largest, richest, and most glorious city in Europe and the Mediterranean region at the time. It enjoyed a cash economy, impregnable defensive fortifications, and abundant food thanks to the availability of Anatolian grain and fish from the Aegean Sea. Silkworms were smuggled out of China in roughly 550, at which point Constantinople became the heart of a European silk industry, an imperial monopoly which generated tremendous wealth. The entire economy was regulated by the imperial government through a system of guilds, which helped ensure steady tax revenues.
Meanwhile, in the heart of the empire, the emperor held absolute authority. A complex and formal ranking system of nobles and courtiers, clothed in garments dyed specific colors to denote their respective ranks, separated the person of the emperor from supplicants and ambassadors. This was not just self-indulgence on the part of the emperors, of showing off for the sake of feeling important; this was part of the symbolism of power, of reaching out to a largely illiterate population with visible displays of authority.
The imperial bureaucracy held enormous power in Byzantium. Provincial elites would send their sons to Constantinople to study and obtain positions. Bribery was rife and nepotism was as common as talent in gaining positions; there was even an official list of maximum bribes that was published by the government itself(!) That said, the bureaucracy was somewhat like the ancient Egyptian class of scribes, men who maintained coherence and order within the government even when individual emperors were incompetent or palace intrigue rendered an emperor unable to focus on governance.
The imperial office controlled the minting of coins, still the standard currency as far away as France and England because the coins were reliably weighted and backed by the imperial government. The emperor's office also controlled imperial monopolies on key industries like silk, which were hugely lucrative. It was illegal to try to compete with the imperial silk industry, so enormous profits were directed straight into the royal treasury.
Constantinople had as many as a million people in the late eighth century (as compared to no more than 15,000 in any “city” in western Europe), but there were many other rich cities within its empire. As a whole, Byzantium traded its high-quality finished goods to western Europe in return for raw materials like ore and foodstuffs. Despite its wars with its neighbors to the east and south, Byzantium also had major trade links with the Arab states.