I agree this option should be activated by default but i see is disabled by default. This has no much sense for me, has more sense that if user do not want receive replies on topic posted they can disabled it.
Why an user should post in a forum, and not receive replies by email? If want so have to turn off the option.
Best things is let admin decide if want this turned on or off by default. Seems an extension exist for this but not sure will work on beta 8, for the moment maybe not.