Major difference between email and IM is that email address is public - anyone can send you anything.
And IM address is more or less private and restricted - you need to approve other party in order to send you messages.
And this, not the latency, makes the difference.
People are using Skype for example in "email mode": sending messages even when they know that contact is offline - it will be read later anyway.
Yet there are more differences:
1. Uniqueness of address. You may have tons of email addresses. But other party cannot be sure what you are reading any of those. While in IM you have one address in their particular namespace.
2. Consequence of #2: Most of IMs keep conversation as single tape - you know that if you or your correspondent did ever say something you will find it in that particular place - on that tape.
3. More or less guarantied delivery and corresponding UI feedback ("his/her LED is green!"). If you see "message delivered" then it in magnitude of times is more probable that your correspondent have or will read it. With emails it is more like a gamble. How many times we heard advices to check spam folder to find someone's message?
But none of those things are logically linked or dominant. Why can't I just have zero latency email that defaults to a prominent instant-message-like UI for all emails without a subject line?
If desired, you could require approval from the recipient to reach this IM-level visibility if sent by a stranger, with unapproved messages appearing in the normal email box (sorta like FB does with non-friends). If someone is sending you too many, you can de-approve them, and you'll just respond slowly like email. The de-approved sender doesn't need to be told they have been de-approved, and the fact that we don't always reply instantly to IMs provides plausible deniability.
This is one of the reasons why Gmail was a game changer when it launched. Most of the other services at the time offered standard inbox one-message-at-time view, where as Gmail showed them in a threaded view.
Huh? My original question is why we can't have the best of both worlds since their is no barrier to combining them. Merely noting that different features are available with different services does not answer this question.
And IM address is more or less private and restricted - you need to approve other party in order to send you messages.
And this, not the latency, makes the difference.
People are using Skype for example in "email mode": sending messages even when they know that contact is offline - it will be read later anyway.
Yet there are more differences:
1. Uniqueness of address. You may have tons of email addresses. But other party cannot be sure what you are reading any of those. While in IM you have one address in their particular namespace.
2. Consequence of #2: Most of IMs keep conversation as single tape - you know that if you or your correspondent did ever say something you will find it in that particular place - on that tape.
3. More or less guarantied delivery and corresponding UI feedback ("his/her LED is green!"). If you see "message delivered" then it in magnitude of times is more probable that your correspondent have or will read it. With emails it is more like a gamble. How many times we heard advices to check spam folder to find someone's message?