I get that the Abbasids brought on an era of multi-ethnic inclusion throughout the Middle East and steered the caliphate away from centralized Arab rule, but why is Khorasan considered especially important to the revolution?
The chief instigator of the Abbasid revolution, Abu Muslim Khurasani (a propaganda name, means nothing more than "father of a Muslim, from Khurasan") was based in the region. In addition, after the Muslim conquests, several garrison cities called Amsars were set up in the conquered lands. Merv was the site of one of these amsars. Ziyad ibn Abu Sufyan, an Umayyad governor in Mesopotamia, sent a large contingent of Muslims to settle the periphery of Khurasan since many people from Mesopotamia, particularly around Kufa, were either anti-Umayyad or staunch supporters of the sons and grandsons of 'Ali ibn Abi Talib (and thus naturally anti-Umayyad). Ziyad wanted the anti-Umayyads out and far away from the center of the Caliphate, as Kufa was far closer to Syria than Khurasan. Naturally the intermixing of Persians and Arabs that occurred in the Persian parts of the Caliphate, the state policy of Umayyads to non-Arabs in terms of status, and an already powerful anti-Umayyad identity fanned by growing discontent and propaganda culminated in the revolution starting in Khurasan.