Redmine

Actually, you can watch/unwatch an entire forum but not a single topic.
When a message is added to an existing topic, all these users are notified:

• project members (depending on their preferences)
• users who watch the forum
• users who have ever posted a message in the topic

Instead, we could add watch/unwatch functionality at topic level and automatically add as watchers the users who reply to the topic. Notifications would be sent to:

• project members (depending on their preferences)
• users who watch the forum
• users who watch the topic

This way, you could unwatch a topic even if you posted a message into it.
What do you think ?

Yep, I think that's the right way to do it - and, to make it seamless, you could

(a) have a checkbox, defaulted to "yes", to watch the topic when you post to it, and
(b) have an upgrade script set the per-topic checkboxes based on who's posted so far (so that, when you upgrade, everyone who was getting those notifications so far will still get them).

Other forums (Invision and, I think, vBulletin) have the per-topic watches set up that way, and it works well.