add column to display replication lag in mongostat and ability to restrict columns displayed. also, when using discover, having control over displaying only secondaries, or only primaries, or only hidden members, or only priority 0/1. also to display replica size/estimate time in oplog, this is useful is resyncing a large replica set that may run out of oplog.

the delay between secondaries and primary being displayed would be very useful in mongostat. also, being able to explicitly define which columns to display so that not all are displayed would be useful as well. reason for this is that with more columns, mongostat barely fits on a 20" monitor.