It would be great if there was a configuration option that allowed you to specify that all index builds should be done in the background. Due to a bug on our part we accidentally started an index build on a huge data set in the foreground. This totally locked up our application and we were forced to do some replication failovers. We've taken additional steps at our end but it would be nice if this was prevented by MongoDB itself. For a 24/7 application once a collection gets over a certain size it becomes really undesirable to do a foreground index.