I have no objection in accepting subdomains as "Regular listing" as long as they are not with free sites (like blogger.com, wordpress.com) Else they have the option of "Featured listing" to get into Best Web Directory In both cases, site should meet the guidelines and must have relevant content.
If the content is useful and rich --provides valuable information to browsers - I can accept even geocities/googlepages I have been studying dir.yahoo and DMOZ --they always have those pages with valuable contents. So who am I to deny them?
Have few subdomains PR ranked higher than domain. Have subdomain that gets 5K daily visitors, while domain gets no more than 500.
As with any site or content, never say never If the quality and content is good then i'll consider it - of course users can simply add the root domain and then add their subdomain(s) and other pages as deep links...