Like every other provider, you will need an user support. To maximize user satisfaction, keep in mind the following.
Phone support is very costly (it requires more people and it makes collective work more difficult).
Already-made sheets for the most common replies are very useful and you can gather them in a FAQ online, allowing you to reply, "Please see the FAQ, section X.Y".
Collective work of the NIC team is specially useful for user support since nobody can know eveything: useful replies are sometimes made from two different contributors.
Even if you try to limit the paperwork to a bare minimum, you will need some administrative procedures. To comply with the openess and fairness principles, they have to be written and followed.
If you have policy-heavy registration rules (such as restrictions on who can obtain a domain name), you will need procedures for the registration, poissibly lasting several days. As for the user support system, a ticketing software is of great help.
If you have registrars and if there is some form of accreditation, you will need a procedure describing the steps for this accreditation.
Functions assumed by individuals in the registry should be documented. Even if, in a small registry, most people fulfill several different functions, it helps to identify the business tasks of the registry.
Example: TODO
TODO: quote Schneier :-) Not only computer security but also, authentication of holders (see sex.com or aljazeera.net).
In some countries, as soon as you manage a database of individuals, you have the obligation to ensure its protection (against illegal harvesting, for instance, a common problem with the whois service).
Among the procedures, you will need rules for the technical management of the registry. For instance, that no work should be considered done as long as it is not documented. Or rules should said that any exceptional action (such as a direct UPDATE in the database) be logged in a proper place (such as a CVS-managed text file).
[5] Except, of course, spam and insults.
[6] Software can help: email readers with thread support like mutt can help a lot to detect unanswered messages. Besides such simple tools, formal ticketing systems like Request Tracker become mandatory once you have more than a very small TLD. They allow to keep track of the questions and of the replies, to match the various messages related to the same case, etc.