The thing about replacing e-mail is that is isn't just a todo list, for many people it's just a receipt box - the thing I keep all my notifications that I bought stuff from amazon. For others, it's still the primary means of business communication.

My work e-mail is largely about communications, with a todo element to it and unfortunately some file storage too. My "home" e-mail is completely different. It's where I get my monthly statements for banks and investments and where my notifications go. When replacing e-mail you would need to service all these components of what e-mail is.

The thing that originally made e-mail so important was it's identity factor. That seems to have withered away as other services have replaced some components of what e-mail was for.

I would argue that e-mail needs to not be replaced, just reclaimed. My e-mail client (web or otherwise) should know that an e-mail in this case is actually just a twitter DM notification and be smart about how it presents that to me. It should know that something from Bank of America is probably something I want to keep, but something else from Bank of America is just marketing junk.

I haven't seen anything that is smart enough to do that on it's own. I don't want to have to deal with creating filters - it should just know. I would totally switch from gmail if this were out there.