>>IMHO, the butterflies of a Chinese victory at Talas probably preempt Temujin being born at all, let alone becoming Ghengis Khan.<<
I'm not sure I would reach the same conclusion. Even though the Tang had a huge presence in central Asia at that time, they still did not control the Mongol steppe. They had no good reason to, other than perhaps the want to control the folks up there so they wouldn't go about raiding them. So Temijin may be born. The question would thusly be, what would his lands be like at that time in this alternate timeline.
The Turks would still be up in the northern reaches of central Asia, north and perhaps west of the Tang controlled areas. The Tang did not control all the way to the Caspian, so surely there would be a route for a Turkish migration south. Though with less of a gap to exploit, one wonders if the Turks might not have migrated westward into Europe as the Huns and Bulgars had done before them. Hard to tell. But the in fighting among the tribes in Mongolia would have still driven the westward migrations regardless.
>>One important consequence would be that Central Asia remains predominantly Buddhist.<<
And yes, that is interesting. The other unknown here is Tibet. In our timeline, shortly after the weakening of the Tang position in central Asia the expansion of the Tibetans had gobbled up control of the eastern reaches of the nothern trade route anyway. That at the same time that Tibet had expanded its influences in nearly every other direction as well. With a strong Tang still to deal with in the east, I wonder if Tibet might not have sought expansion westward into Arab held lands. Especially if the caliphate had suffered a major defeat by the Tang shortly before.
I have to wonder if the outcome of a Tang victory at Talas might have generated three results.
1) A strengthening of the hold the Tang had on the region and possible expansion of their holding to extend all the way to the Caspian.
2) A major expansion of Tibetan power westward as well. Absorbing the Afghan reagion and perhaps pushing the Caliphate backwards to the modern day borders of Iran. I still rather doubt the Tibetans would have pushed much into India, but rather simply have taken advantage of the weakness of the Caliphate.
3) I suspect the Turks would have immigrated into the Russia steppe rather than into the Caliphate, simply since the later may have proved more difficult with the Tang still in a firm grip of the easy route south.
The net result would likely seem another round of trouble for Europe and possibly a forestalling of a possible formation of Kiev Russ. On the flip side, without the Turks to come crashing through all the way to the Caliphate's core, in spite of their lose at Talas and further losses to Tang and Tibetan expansion, they would probably be better off for it.
If that's true, do the internal politics of the caliphate still rip it apart?
Does this lead to a Tang - Tibetan war for ultimate control of Central Asia and its trade routes?
And come his time, will the Tang control of the region and its subsequent influence on the Turks to their north, does friction with the Turks still lead to the political environment in which Ghengis Khan can unite the Mongols? If such a unification is still possible, the Mongols could still represent a major threat, but with China both united and strong I have to question if the Mongols could have succeeded against them, or would even have dared to try. If not, perhaps Ghengis Khan might not have achieved much more than a modest steppe empire.