The Internet Governance Project did an analysis of the ToS of 74 registrars who manage 90% of all domains. Their findings are intriguing especially since they show that there is a link between domain name policy and internet content regulation. 

e.g.:

We found the ToS of 26 registrars using the term “moral” in some manner, ranging from prohibiting use of the domain that is “contrary to” or “violates good morals”, or requiring that the domain or content found at the domain "must comply with...social public morals” or “good morals", to the most frequently found restriction of domain name use in "morally objectionable activities". 

Might be of interest.