What differed in the organisation, structure, doctrine and influence of the Orthodox church that prevented it from being the victim of large, game-changing reform movements like those of Lutheranism and Calvinism seen in the Catholic church? Was it largely a matter of geo-politics, i.e. the Orthodox church prevailing in, more-or-less, just Russia, as opposed to Catholicism being spread across hundreds-upon-hundreds of culturally different states all over Europe?
I think it would be difficult to establish a specific reason, but two factors that were probably important were:
A) that the national Orthodox churches maintain some degree of autonomy, whereas in the Catholic Church the center exerts a much larger influence. The local Orthodox churches were more flexible and more easily able to deal with local challenges.
B) that the national Orthodox churches are national. The religious identity and the national identity have some amount of overlap there. It's not a church controlled from some foreign remote location, it's described in those countries as "our church". The combination of religion and nationalism is always very powerful.
I'm speaking mostly about East European countries, although Orthodoxy is not limited to that area. Also, those two factors alone are probably not sufficient.
It's also important to remember that, at least in Russia, there was a prominent era of reform under Patriarch Nikon (1605-1681) and later Peter II, but we don't really think of it in the same way, since it was imposed from the top down by the most powerful men in the Empire; widespread opposition from the Old Believers aimed at preserving traditional church practices. This was the Raskol, or 'cleaving apart', and persecution of the Old Believers continued for hundreds of years; anathemas placed on the Old Believers are just recently being lifted.
While Orthodox church is indeed different from Catholic church in a lot of ways, it is not exactly true that there were no reform movements.
Probably, the biggest example is the Schism (17 century), also called by the Russian word for it: Raskol.
One might also consider Greek Catholic churches a reform movement (16 century, started with the Union of Brest).
Depending on what you want to call large-scale, it might also be interesting to look at sects like Khlysts and Skoptsy.
Interesting answers so far, but I should clarify I mean reformations in the sense of ones that broke off and established new churches, like in Germany/Scandinavia/Britain. The Orthodox churches on the other hand, seemed to remain intact and as unified institutions.