We support commenting inline on statuses using the FBSS Comments module, and there is an issue to #1286474: Allow commenting on nodes from the activity stream, but it's possible that someone might want to comment on other things (like changing profile pictures?). We should support that. However we want to make sure this doesn't conflict with status or node comments, so we need to make sure that they are in sync.

We want to allow enabling or disabling comments on any activity message type. However, Heartbeat does this, and it's extremely confusing that you enable the heartbeat comment module and nothing happens -- no comments anywhere. So I think the default when the future activity comments module is enabled should be that activity comments should be added to every activity message type (which should be the usual desire) until explicitly disabled, and we also need the messaging to be very clear that you can disable them per activity message type.

Activity comments should support everything FBSS Comments do -- showing a configurable number of comments inline, expanding when you type in the comment box, etc.