I have no idea what w:majority is supposed to mean in a replica set where only arbiters can vote, but it can't be good. When I tested this, a w:majority write was acknowledged by the primary even though all the secondaries were down at the time.

Since such a replset config is highly likely to be (a) unsafe and (b) a mistake, we should output a warning if this situation is detected at replset initiate/reconfig time. We should also check to see if there are any other strange replset configs that users should be warned about.