When one assigns a task to some person, say A, and realizes that this was a mistake and assigns the task to person B instead, A stays a follower of that task. That is something one can very easily overlook. In some cases, A gets notified about things he/she does not need, or even should not see at all.
So here is a feature request: Asana implements that, when a task is created, an assignee is chosen for the first time and then within, say, 60 seconds a different assignee is chosen, the previous assignee (in case (s)he is not mentioned anywhere else in the task) is no follower anymore. What do you think?
Of course there are some cases in which a deleted assignee should stay a follower, but I think that these cases can be ruled out by the specification above.