A server had a failure in the middle of the night that caused a hard-reboot (nothing to do with Mongo) but then after boot the MongoDB service never started up again (turning the 1-minute of downtime into most of the night until a human would intervene.)

The issue was that the Mongo server would not start up unless all locks are cleared, this was in the log:

• • • • • • • • • • • • • • old lock file: /data/db/mongod.lock. probably means unclean shutdown
                            recommend removing file and running --repair
                            see: http://dochub.mongodb.org/core/repair for more information
                            *************

This seems great if you are restarting and really bad for a fresh-start when Mongo is not currently running. My startup script now has a clause for:
mv /data/db/mongod.lock /data/db/mongod.lock.old

But this is a hack - the Mongo server should just ignore the locks and start-up if there is no server currently active.