Ottoman mistreatment of Christian minorities is a trope of the nationalistic Balkan historiography written during the nineteenth and twentieth centuries. While in recent decades professional historians in the Balkans are generally coming to terms with their Ottoman past, the notion of Turkish oppression continues in the popular imagination. Some Balkan school books taught that Ottoman rule consisted of literal torture and slavery for the population, and in general distorted history to a degree which western historians have never accepted as accurate.
The Ottoman Empire ruled the Balkans for many centuries, and Balkan Christians naturally experienced a very wide range of treatment at the hands of their overlords. It's always possible to find isolated instances of mistreatment and abuse, especially during the latter years. However, historians generally have a positive view of Ottoman rule vis-a-vis their minorities. The Ottomans were, comparatively speaking, more tolerant toward religious minorities than Europeans were during the early modern period, offering them the freedom to practice their faith in exchange for the payment of a special tax (cizye), the rate of which varied quite a bit by time and place and thus cannot be said to have been a universally light or heavy burden. Balkan Christians were also subject to the devşirme, the so-called "levy of boys" whereby the sons of Christian families were occasionally conscripted into imperial service. It is very hard to determine what the average Balkan Christian thought of this practice - many probably looked upon it with horror, others saw it as their rightful duty to their sovereign, and some wanted their children to be conscripted in order to give them a higher status in life as part of the ruling elite.
One important account which sheds some light on Balkan perceptions of Ottoman rule is that of Papasynadinos, a priest from Serres writing in the mid-seventeenth century. In his Greek-language chronicle, he never questioned the legitimacy of Ottoman rule, referring to the sultan as basileus in the style of the Byzantine Emperors, and attributing positive qualities to many individual sultans. To him, the Ottomans were certainly not an oppressive, invasive force to be resisted and fought against. They were the legitimate rulers of the land.