To put it simply, Gmail accounts are very insecure. We've had some problems in the past with existing users losing their identities when others infiltrate their Gmail accounts and steal activation E-mails, so that domain has been disallowed.
I'm not sure if any other domains have been disabled; I don't recall any offhand. If there are others, then they likely include Yahoo and Hotmail just because of the ease with which you can create accounts. But I don't believe those are banned; Gmail is just notorious for its susceptibility, so it had to be done.