We are constantly getting into various situations then user should't be able to do something before he do something else. For example, you shouldn't be able to set task to a 'final' or 'closed' as long as you have Open Notes linked to it (or to it's related versions). Status field to that state should be disabled until you close all the notes. And so forth. So, it would be nice to have rules for setting conditional restrictions based on other field(s) state(s).