The Middle East and North Africa (MENA) region is in crisis in part due to western policies of regime change and to the imposition of western values on others with no understanding of or respect for local contexts. Furthermore, other views on the region, like Russia’s, are ignored. The destruction of Iraq was the basis for the rise of extremist non-state actors. From there the region’s implosion was propelled by additional military activity or support for separatist groups.
It is most important to note that the so-called “Arab Spring” has led to catastrophic outcomes and the proliferation of terrorist groups.
In this context, Russia's priority is to develop a coalition to defeat terrorism.
Moscow is willing to cooperate with any like-minded country and is channeling its efforts to stabilize Syria through the UN and the Geneva process. The U.S. is not willing to support these efforts.
On Turkish policy regarding Syria: from the beginning, Turkey believed that Assad's regime would fall. Suitable policies followed, but the assumption turned out to be incorrect.
These policies have been changed in light of the developments over the last four years. The current policy now is to fight ISIS and "other" opposition groups in northern Syria ("other" refers primarily to Kurdish forces, because if the Kurds win they'll create a contiguous presence along the Turkish-Syrian border, which would be unacceptable to Turkey).
The current Turkish interest is in the preservation of Syrian territorial integrity, which coincides with the position of Assad, along with Russia and Iran.
In the current revised policy, preventing the creation of Kurdish territory along Turkey’s southern border is the priority. The Kurdish threat is seen as an existential one for Turkey, whereas ISIS is not seen as a direct problem for Turkey.
There are huge disagreements between the U.S. and Russia on Syria, including: most in the U.S. see Russia's policy as deliberately targeting the civilian populations in Aleppo; there are disagreements about the future of Assad; while the U.S. prioritizes the fight against ISIS, Russia is fighting other groups, including the al-Nusra Front (this summer, al-Nusra Front changed its name to Jabhat Fateh al-Sham).
There are other differences as well but these are the top ones.
There is a path forward, but a lot must be done by both sides to reengage.