What Should and Shouldn't be said over E-Mail?

The other day, someone I know told a story about one of her good friends getting dumped by her boyfriend via email.  Said couple had been dating for a couple months.

Now, a mere five years ago, this would have been considered horrific form.  (Some might say it still is.)  However, I will venture an educated guess that things like this taking place over email are not that uncommon.

The question of 'is it ok to break up with someone over email' comes up, naturally.  However, aside from just that - I think a lot of serious personal conversations take place in this medium.   It is a nice easy way to address situations that require some level of confrontation with minimal discomfort for both parties involved.

However, something is obviously lost.  There are several problems - we don't see facial reactions/emotions.  People have time to think about and edit their responses - they can even ask others for input.  Also, there isn't an absolute obligation to respond to an email, whereas with a face-to-face conversation (or even a phone conversation), avoiding responding is much more difficult.  And, perhaps the biggest problem of all - there is a 'forward' button.

However, since so much conversing takes place over email, do almost all important confrontations nowadays take place this way?  Have a lot of personal, important, emotional conversations occurred in this medium?  Is this a good thing?  I'm inclined to say no.  It might seem like an easier way for one to say what he or she needs to say that one might not have the nerve to say to someone in person, several things are definitely lost.

Where is the line drawn of what can and cannot take place over e-mail?   Are all awkward conversations surely migrating over to email?  How does one tell someone it's inappropriate to talk about a certain topic via email?  Your thoughts?