Email is great because, at its heart, it's a super-simple Unix-ish concept: send text-based messages to another address. (Granted the history and implementation has grown quite gnarled, but that's not important for day-to-day UX.)<p>That simplicity is what makes it so compelling: you can twist email to be whatever you want. Some use it as a to-do list, some as a calendar, some for editing documents, some just for messaging. Everyone can use it in a different way because it's so simple and easy to understand.<p>One can certainly invent products that try to take one aspect of email and improve them, like a to-do list, but then it's not email anymore... it's a to-do/calendar app. Which isn't bad, but the point is--email isn't going anywhere.