Why do I have to run mongod as root?
First of all when you run mongod as a normal user you need to make sure that the particular user has the following permissions:


Access to network interface ( open socket ) in order to be able to listen to connections
Access to the database folder. The default is accessible only by root or mongod user (/data/db)
Access to logging folder /var/log/mongod


All of the above are automatically set and granted for the user mongodb (mongod in earlier versions). If you take a look at /etc/init.d/mongodb you will see that the service will be stared under the user mongodb DAEMONUSER=${DAEMONUSER:-mongodb} and not as root.