Disabling a feature is best applied when it shows the user that they can do something, just not right now. Two examples are menu options that aren't available, or a data entry field that they can't edit. This could confuse users if done in the wrong context. For example, if it's a form field that has information that is defaulted and they can't edit it, why use a disabled form field? Just display the information as text.
I've always tried to focus on simplicity – if the user can do something, show them what they can do. If they can't, then don't present the option at all.
I think the best example of disabling is if you are in a stepped process or "wizard" – show all he steps, but disable ones the user hasn't done yet, because often these processes are interdependent – you can't do step three before step one because there's information needed in step two.