I sometimes get emails (e.g. from professional contacts or people I don't know well) which simply start with
FirstName,
[ ... letter body ... ]
They don't use "Dear FirstName," or "Hello FirstName," just "FirstName,".
This always feels quite harsh to me. (Omitting the opening completely in a quick email wouldn't feel that way, but using the name only does.)
Can this be considered impolite? Is it bad form? Is it usual? What should I read into this (about the attitude of the writer)?
I realize that these things can depend more on the culture than the language. I usually get these letters from Americans. Is it the most usual way of opening an informal email in the U.S.?

first name,– that's it. If he were to add some word in front, I might think I was in trouble! I suppose it all depends on what you're accustomed to seeing. – J.R. Jun 15 '12 at 10:50