I think that the user registration provides better long-term relation between user and site than an e-mail entry on comment or ticket. For example, the problem that I see with t.e.o: - e-mail changing is not possible for submitted comments - if comment was submitted with email, the user cannot edit it.
Supporting both ways is also, IMO, is not good: imagine that user once sent a feedback with email and another time as registered user - things can be confusing. So, my 2 cents to require user registration, but make it simple and clear (may be with support of openid or/and google accounts) and with subsequent redirect to the original url e.g. ticket creation url. Cheers, Andrej
