There seems to be no button to subscribe to a topic when I am replying, or an auto-subscribe. Is that intentional, or can it be changed?

An auto-subscribe would be a bit too overkill, since too many people would activate it without reason. On the other hand, I see no reason not to have a checkbox to be subscribed to a topic when you create it, or if you reply to someone else. I'll add this to my list of upcoming features.