When I need to reach someone, I usually want them to respond as soon as possible. That's why I'm trying to find a way that I know they receive. Sometimes it is skype/gtalk (if i know that the person is at a desk with clients online), sometimes it's sms (if i know that they have their phone). And of course, voice call, if text doesn't cut it.

If I know their main communication agent is email and it's business hours, I will send an email.

My point is, you can make a perfect replacement for the email, from point of view of recipient, but people who are in control most of the time are the senders.

THEY will choose a way to contact you. If they see that FOR THEM it is more efficient to send a chat, SMS or email, they will do it.

Of course, you may try telling them: "That's it, I'm using only my new perfect todo tool", but I don't think it will hold for long.

I may definitely be totally wrong.