I'm used to creating "virtual servers" in CPanel/WHM. I'm not sure if the same options that WHM has are available elsewhere in Virtualmin, but I'd like to see options to:
WHM also allows me to restrict quotas (disk space, ftp accounts, subdomains, etc.).
Maybe this is part of setting up Virtualmin as a reseller configuration and I just haven't looked into this yet.
Anyway, I'm not going to be reselling on my Virtualmin server so I don't care too much about the quotas, but from a security point of view, it would be good to easily disable cgi access and shell access for some of my domains (so someone just can't guess a password or upload a cgi script somehow and hack my system).