Where are the logistics components, like fuel and munitions, necessary to conduct a sustained campaign going to come from and how are they going to get to Syria?
Russia does have a large naval and air base in Syria.
The combat aircraft and other hardware gets all the headlines because they are the most visible and high profile, but it would make little sense for the Russians to not also stockpile munitions, and especially fuel at those bases.
Those can be shipped in either by air or by sea.
In addition, I think logistics and support would be a key area that China would be very well suited to providing.
I have a feeling that there are very intense diplomatic efforts going on behind closed doors right now, as the Russians would not like to be out acting unilaterally any more than America.
They will do if they have to, but they would much rather also have the diplomatic and material support of a powerful ally like China.
Which may be why they have been holding off on actually launching any air strikes, and maybe also why some in the west are trying to almost push Russia into making a move prematurely and alone. Probably so they can launch a vicious and sustained PR and diplomatic offensive against the Russians in the hopes of being able to warn China off from joining the Russians.