Something of a national topic, but I'm curious.
Obviously, by 1800, let alone 1900, the Balkan states were much weaker and less developed than Western and Central Europe. But how much of this was due to the Ottomans, and how much of this was due to preexisting conditions in the region?
In other words, if you replace the Ottomans with a Byzantine rump state in Greece, a Bulgarian Empire, etc., would the region have been more developed?
The Ottoman Empire was more tolerant and flexible than it's sometimes given credit for...but only to a certain degree. The consequences of its conquest and rule of the Balkans were very negative.
First of all, the natives had huge demographic losses to pillaging and enslaving during the conquest. It's not like the Ottomans invented pillaging, but they did incorporate plunder as a method of warfare in a major way (with the Akinjis and all). And slavery had barely even existed in the Balkans before the Ottoman Empire brought it in big time.
The other thing is how the upper classes and most of the middle classes of the (Christian) natives were wiped out, practically reducing entire peoples to an oppressed, 99% peasant, underclass.
The Ottoman Empire (assuming you live in the interior and not on the front lines with all the slave raids) wasn't that bad at first - there were "special" social categories through which quite a few Christians could acquire tax exemptions and other benefits, and some lesser nobles were even allowed to keep their position within the new system; the Empire frequently appointed Christian intermediaries between the local population and the ruling Muslim class, which helped limit discrimination.
But, while some empires get more tolerant and open as time goes on the Ottoman Empire
lost much of its (relative) tolerance in the 1500s.
Fewer and fewer Christians were given the "special" rights and benefits, the lesser Christian nobility was ousted in favor of a fully Muslim ruling/administrative class, and the system of intermediaries was gradually reduced or abolished outright, exposing more and more Christians to discriminatory laws they might have previously been barely aware of. The central government accepted less and less petitions and was eventually completely supplanted by the rise of the privately owned Chiftliks and strong local governors who had much less scruples about exploiting and mistreating the local Christians.
The Tanzimat was a decent effort to re-centralize the Empire and bind the Christian natives to it with social reforms, but by that point the discrimination was so ingrained in the administrative structures that the reforms rarely managed to push their way further than the immediate vicinity of Konstantiniyye.
Another way the Ottoman Empire screwed things over were the way various ethnic groups were moved around during its conquest and rule over the Balkans, creating hotspots of conflict which persisted even after the Empire itself left.