There have been many suggestions along this line already. My view is that it would be useful to know, as the OP:
- When an answer is edited;
- When an answer is added (done);
- When a comment is added;
- When someone edits the question;
- Arguably when a comment or answer is deleted.
As an answerer you are generally interested in:
- When the question is commented;
- When the question is edited;
- When your answer is edited;
- When your answer is selected (done);
- When your answer is deselected;
You can also make a case that an answerer could also be interested in:
- Edits to other answers;
- New other answers;
- Any answer being selected or deselected;
- Any comment.
Also you could argue that someone who is just reading the question is interested in some or all of the above. This is particularly the case when you find an interesting question that hasn't been answered yet. The only way of dealing with this currently is to favourite it and remember to go back to it, which is fairly crude.
Lastly, all of the above should be able to be treated as a subscription (ie you can unsubscribe from some/all notifications).