Replica sets provide great automatic failover and redundancy, but the failover in any relatively high traffic database is almost useless since the secondaries don't seem to keep indexes or recently queried data in memory. We've solved this in the interim by writing a script with slaveok to query in the indexes and "warm the cache." I consider the fact this is necessary is ridiculous in a production server.

Could MongoDB secondaries please keep hot-data in memory so that in the case of a failover, page faults don't skyrocket. At the very least, indexes should always be kept in RAM.