Originally the reIndex command accepted a write concern. But it turns out that was a mistake and the latest versions of the server no longer accept a write concern for the reIndex command.
The driver should stop sending a write concern with the reIndex command.
For 2.4.3 we are simply going to ignore any write concern passed to ReIndex methods.
For 2.5 we will deprecate the WriteConcern property of the ReIndexOperation and for 3.0 we will remove it.