Validator parameters
You can specify multiple Issue Fields (both default and custom) to check. You can also check Fields that appear on transition screen (e.g. forcing the user to write a comment while transitioning an Issue).
Use buttons with arrows to move your chosen fields to the "Selected Fields" section.
Example: Image on the right shows a Validator set to make sure the Issue has a user assigned to it, a Custom Date Field filled and forces the user to write a comment and attach a file when transitioning an Issue.
Displaying a message
You can change the message that will be displayed for the user when Transition fails to satisfy Validator requirements.
You can also add $$FIELD_NAME$$ and $$FIELD_ID$$ inside your message to display empty Field's name and ID respectively.