You're talking about the same kind of "normal people" who will reply to noreply@ anyway and complain that the company doesn't respond. In that case, how does it help anyone if you keep using a noreply@ address, whether for privacy or for any other reason? If "people still just reply", maybe the right thing to do is to make those replies actually work, instead of trying to restrain normal people's normal behavior.
It doesn't make any intuitive sense that other people's replies to what you wrote are delivered to you by e-mail but you can't use e-mail to reply to them in turn. That's only half of a proper communication system. Give people a proper communication system, or give them none. If you only give them half, it is only natural that they will expect the other half to work. "Normal people" simply don't care whether SMTP is different from HTTP.
You say it'll be very difficult to make e-mail replies go back to the conversation thread on the web site, but Google Groups and Posterous group blogs have been doing that for years. Google even detects quotes in replies and hides them automatically; you could do the same in your forum software if you want. Support ticket systems of every web/vps/server host I've used also work with both e-mail and web. There are some kinks here and there, but integrating e-mail into a web conversation is a solved problem for the most part. There's no reason for e-mail notifications nowadays to be not reply-able, unless it's a one-off thing that clearly does not create any expectation of reply-ability.
It doesn't make any intuitive sense that other people's replies to what you wrote are delivered to you by e-mail but you can't use e-mail to reply to them in turn. That's only half of a proper communication system. Give people a proper communication system, or give them none. If you only give them half, it is only natural that they will expect the other half to work. "Normal people" simply don't care whether SMTP is different from HTTP.
You say it'll be very difficult to make e-mail replies go back to the conversation thread on the web site, but Google Groups and Posterous group blogs have been doing that for years. Google even detects quotes in replies and hides them automatically; you could do the same in your forum software if you want. Support ticket systems of every web/vps/server host I've used also work with both e-mail and web. There are some kinks here and there, but integrating e-mail into a web conversation is a solved problem for the most part. There's no reason for e-mail notifications nowadays to be not reply-able, unless it's a one-off thing that clearly does not create any expectation of reply-ability.