How amazingly far apart are the tones of e-mails that begin with "hi" and those that begin with "dear." My use of "dear" is hard to break because I grew up being taught that one began letters with that word. But I can't help noticing that this usage makes me seem archaic, so I increasingly resort to "hi" in ordinary correspondence.
It is too informal for my taste, though, or rather it is too detached and chilly. It is like running into someone in the supermarket by accident while searching for spinach ravioli. "Oh hi (so and so)," one says without stopping or scanning the shelves. Interestingly, we in this country tend to associate formality and formalisms with emotional coldness, but I think informality can be just as if not more dismissive and off-putting. This country's modern economy and culture is largely founded on emotional detachment. In a word, on "cool."
I find my brother R, for example, to be most formal and ritualistic when he is most affectionate, such as when he presents a gift with a little speech telling where he saw it and how it reminded him of you. Formality may be the resort of the emotionally distant, but at least it is clear -- more clear than a lackluster, monosyllabic "hi."
The scope of "dear" has shrunk I think from older times, when it was used in a wide range of emotionally charged contexts. Now it has narrowed considerably to opposite sex romantic relationships. But I still will use it in e-mails with men, if I want to mark it -- and them --- as important.