We are now in the process of switching to Mongo DB 1.6.1 in order to use Sharding and Replicat Sets.
Yesterday, I found out that when Mongo was printing:
I opened a bug at Mongo (http://jira.mongodb.org/browse/SERVER-1637) and Eliot pointed me to a fs configuration change that could help.
We have now opted for XFS local disks (instead of 8 RAID-1 EBS disks in Ext3). XFS is supported by the kernel of the instance we use while Ext4 is NOT.