I was surprised to find the "Unsubscribe" button on Special:TopicSubscriptions wasn't a JavaScript-enhanced button but a simple link. I expect it to behave exactly like the "[ unsubscribe ]" link on discussion pages—i.e. disable itself, send a POST request, show a notification—all without leaving the page. Upon success, the row should be either hidden or grayed out (the latter is probably more humane as it makes undoing easier).