Jon Udell has an essay on the value email has as a social substrate, value which is not encapsulated in other tools:

An ad-hoc group convened by e-mail dissolves unless membership is reaffirmed by each message. This is a feature, not a bug. Many of the groups that perform work in a modern organization are transient. A hallway conversation is over in minutes; a spontaneous collaboration can last a day; a project may take a week. Software that requires people to explicitly declare the formation of these groups, and to acknowledge their dissolution, is too blunt an instrument for such ephemeral social interaction.

Amen. Email is too good to be trivially replaced, or, put another way, anything that had all of email's good features would be email. It may be that the bad features of openess inevitably lead to the demise of open systems, and if so, let us all weep for the future, but even if that happens, that is not the same as asserting that email can be simply replaced by something else. One quibble: The hallway metaphor is apt, but not perfect. Anyone who has been on a "I had to CC 34 of my closest friends to discuss this Very Important Matter" knows that email can also include you in a discussion you'd really rather not be part of, and unlike the hallway conversation, you can't leave. (Someone, I forget who, called this pattern "to unsubscribe, just die.") There is a very interesting space between the low-overhead, short-livedness, and poor user control of CC line conversations vs the relatively high setup costs, long-livedness, and good user control of mailing lists, a space that might be occupied by software that easily converted CC line conversations to mailing lists that would let the users unsub, but would vanish unless the users periodically re-ratified its existence.