Drag
The atmospheric drag up there at 36,000 KM is negligible, but such a large mirror might start acting like a solar sail. Compensation for that should be possible by careful orientation, which can be handled by a couple of gyroscopes. And the power for the gyroscopes can be generated by solar panels. Just make sure those gyroscopes are of a high enough quality (no China Export) or the satelite will be as usefull as Chandhra currently is.
Biting the hand that feeds IT
