I believe a notification that a question has been edited or re-tagged would be very useful. If I am in the middle of answering a question that is simultaneously edited, I might like to know whether the parameters have been redefined.
For example, I was in the middle of answering How to determine that there is date in the string or not with different date format? when the question was significantly changed. The OP rolled those changes back and asked a new question (kudos to him), but not all users would be so gracious.