Currently when you run db.currentOp on a mongos, it tells me which shard it is running on, but not necessarily which node (replica set member) it is running on. This would be particularly useful when running aggregation framework and mapreduces on secondaries to:

A: Confirm it is following your read preferences and running in the correct place.

B: Be able to kill these jobs when they are executing on secondaries.

Currently, it only gives the current operations on the primaries of each shard and doesn't allow you to kill ops on secondaries without logging on directly to that secondary.