The early consequences of this far-flung expansion are paradoxical, and depend on exactly which geographical area one occupies when forming an observation. Within Mecca, Medina, and Syria, centralized political control had left its mark. Vast amounts of income poured into already-well-connected trade communities. The rapid ascent of the ruling body promoted tribal conflicts and in those brooding subordinate tribes, resentment festered slowly. The successes of the Arab conquests, however, added extraordinary credibility to the Islamic ideology, and thus complicated the matter of removing the Quraysh family from power. On the other hand, the most powerful military machine in history offered a phenomenal incentive and potentially unlimited career opportunity for Arabs who had long suffered limited prospects under the shadow of the Roman and Sassanian empires.
After the passing of the Rightly Guided Caliphs, the Umayyad family exploited existing political and religious divisions to seize power, shifting the center of administration to Syria, under Khalipha Muawiya, from Medina. Due to the role of Muawiya's son in the death of Ali's Heir (Ali was assassinated, like the caliph before him), tensions escalated, and an opposing political party began to take shape. For Arabs, life couldn't have been better, in spite of the complex political evolution. The ones who reaped the newly created wealth became richer than they ever imagined. For their conquered, it would seem that life improved in important ways as well. The Dhimmi, or people of the book, were treated with tolerance, and could expect civil protection if they agreed to pay tribute. Everyone else was given the opportunity to convert, and become clients of the Arabs, or Mawali. Many of these enjoyed some greater degree of privilege and a rare opportunity to be welcomed into the society of conquerors, instead of being exterminated.
Additionally, and especially for those regions in north-Africa and Anatolia, though the Byzantine overlord was merely replaced with an Arab, this tolerance, coupled with a reduction in tax burden, incentivized cooperation, which eased the administrative burden. Success encouraged war torn local states to support the Arabs, and tolerance maintained easy relationships with the client regions.
Under the Abassid empire, Islam grew, but the empire would slowly attenuate as the border enemies regained their confidence. Quality of life and education soared for a time, however, and the House of Wisdom in the new and booming capital at Baghdad had the time and security to reintroduce ancient Greek thought, and to make irrevocable advances in math, astronomy, medicine, and art. The cosmopolitan life was ill suited to the Arab spirit for the period, however, and the consequences of unchecked internal conflict and ungovernable frontiers brought calamity as Abassid administration deteriorated under the weight of opposing interests. The Turkic people were tapped to provide security to the governing body, and they turned the caliphate into a sultanate. Fatimids took Egypt from the Abbassid, exploiting their vulnerability to outlying rebellion.
The bright side of this sad turn is that displaced Muslims carried their cultural contributions outward from the comfort of the fertile crescent, and as a result, Islam gained strength in the more distant areas it had reached. Great new metropoli, such as Basra and Cairo, rose to international promenance.
These outcomes would factor greatly in the preservation of Islam against the challenging adversity it would soon face from the west.
Image Source: http://upload.wikimedia.org/wikipedia/commons/2/2c/Maqamat_hariri.jpg