The point of etiquette is to smooth over social relations by providing a bit of uniformity—like paving a roadway. With that in mind, Nick Bilton’s advice  in the New York Times on whether, for example, to send brief thank-you emails is not terribly helpful: “think of your audience. Some people, especially older ones, appreciate a thank-you message. Others, like me, want no reply.” Well, yes—if you know in advance what somebody’s preferences are, you can tailor your correspondence to suit them. Etiquette is what you need when you don’t know and don’t want to risk embarrassment by making a wrong guess.
It seems to me that thank-you emails fall into a realm where proper etiquette is not to take offense or have expectations either way. It’s certainly snotty to get upset about someone thanking you, even if a brief email conveying gratitude is a bit of time-waster for everyone. (Most etiquette is somewhat wasteful; it’s not purely functional.) But even most business email is informal and utilitarian, so don’t take offense at not being thanked, either. Forbear until convention—and attrition—resolves the ambiguity.
Bilton notes that “opening an e-mail with ‘hello’ or signing off with ‘sincerely,’ are disappearing from the medium.” That’s fair enough, but I still use a salutation and a friendly sign-off, not for the sake of an old convention but to set a tone. Opening with an informal “hello” or “hi” and ending with “best regards” or “all best” is a way of softening whatever blows may fall in the body of the correspondence. Without such pleasantries, rejecting an article or calling for extensive revisions, or even conveying good news in businesslike fashion, can sound very cold. And too much fluff in the body of a message creates confusion—best to open with a warm handshake before getting down to business, to keep clear the difference between personal esteem for a correspondent and the hard facts of whatever is under discussion. With personal acquaintances that stuff is unnecessary, but it’s good business to set people at ease, even if the form for doing so is a little old-fashioned.
On the other hand, I completely agree with Bilton’s advice against leaving voicemail. Someone too busy to read an email won’t be very happy to take a call or listen to a voice message. There are times when Alexander Graham Bell’s invention is indispensable, but dispense with it whenever possible, and don’t make it your first recourse.