In my particular situation, I have a comment form than cannot be used until the user has logged in and joined a specific topic. Then, they can comment on that topic.
In your opinion, should I hide the comment form completely, or disable it and prompt the user to complete the necessary requirements.
Disabling them saves the user from effortlessly looking for functionality she knew to exist before. So in general, hiding something completely from view just generates frustration. Remember the dynamically populated menus in Office 2000 to 2003? Then you know what I mean (cf. Jensen Harris' blog posts on that topic).
I suggest you should disable them and make it clear in what states they are available and how to achieve that.
In some cases, however, such as the application we are developing right now, functionality being there or not depends more on the user's permissions than on the current state of the program. In such cases it can be helpful to just hide things that shouldn't be accessible since users never get to the point where they could use the controls. Simply because they're lacking privileges. See for example Stack Overflow's moderation tools which are accessible once you get above 10k rep but are never shown before, not even as disabled.