Personally, the "Undo Send" feature is one of my all-time favorites in Gmail. A lot of people are comparing it to an outbox, but while the two are functionally similar, they are psychologically different. The undo send simply gives you a few extra seconds to re-read your email and catch any small errors before sending.
When the feature first launched, I was worried I would eventually adjust to it, and stop re-reading emails before clicking "send". So far, that has not happened - it's effectively a psychological trick that continues to make my emails better proof-read.
This point was raised when Gmail originally launched the feature. The problem is that it would essentially act as a read receipt, which is a privacy concern.
Plus it would add to confusion - you see it in your inbox (unread) and then you don't and wonder what happened. With Exchange you can do this and it usually just gets ugly in practice.
It would have to be based on any interaction the receiver has with the email, including viewing the inbox. However, if we're already worried about that level of privacy, even that change would do no good since it would mean the sender would know when the receiver checked their inbox.
I tend to use Drafts more for emails that aren't actually completed, and an outbox for emails that are completed but with one last chance for me to look them over before they go out.
An interesting idea for a web-based email site. I don't think I've seen one (not that it really makes any sense, but I can see how some people would like it). Closest you have are drafts, which are pretty darned close, but it's not quite the same, no.
I still fall on the "don't send it unless you mean to send it" side, however. My outbox is only used when I don't have internet access.
Offline Gmail certainly has an Outbox. The main reason being you'll generally have no internet connection or a very flaky one. By product of it is that yu can indeed cancel emails after sending them.
It wouldn't be a bad idea to use an Outbox for some people. Gmail could offer it as a labs feature and whenever you request or end your session/logout it would send your email...
Removing email from an account once sent like above seems like a very bad user experience quirk.
When the feature first launched, I was worried I would eventually adjust to it, and stop re-reading emails before clicking "send". So far, that has not happened - it's effectively a psychological trick that continues to make my emails better proof-read.