Fashions, of course, change, but change slowly. "Dear" in a salutation carries no overtone that the person is in fact dear; it is a pure convention.
There was a time--and it continued until the 1940s or so in the U.S. Department of State, that the most formal letter began "My Dear Mr. (or title)." That sounds even more intimate that a simple "Dear ..." but it was then more formal.
In the United States, the "Dear ..." would be followed by "Mr." or "Madame," or "Dr." or "Rev.", or the title of the position. (Dear Senator x; Dear Congresswoman Y ..." But with the President, Vice President, and cabinet members it would be: "Dear Mr. President; " Dear Mr. Vice President; Dear Madame Secretary (if you are addressing Dr. Rice), or "Dear Mr. (Madame) Ambassador." These would be followed by colons (

, not commas. (If the letter is personal, the salutation is followed by a comma.)
In addressing the envelope, however, the address would read (except for the president and vice president) "The Honorable ..." (There is a story about the Kentucky Colonel who was asked by his Senator just what the title Colonel meant. He replied, "It's like the Honorable they put before your name; don't mean a damn thing!")
In Britain, I believe, they leave off the Mr. or Madame and just write "Dear Prime Minister ..." Orally they address the Prime Minister as simply "Prime Minister," whereas Americans would say, "Mr. President," or "Mr. Secretary," etc.
In formal typed correspondence, one would never write "Dear John" or "Dear Mary." The typed salutation would be "Dear Senator Evans ..." If, in fact, the Senator is a personal friend, you would cross out with your fountain pen the "Dear Senator Evans" and write in your own handwriting, "Dear John,"
Of course, in personal correspondence, one would use the first name if that is what you call the person when you meet. However, I--and many others of my generation--consider it tactless and presumptious if someone we do not know addresses us, either personally, on the telephone, or in a letter, by our first name.
For emails I use Dear ... if I don't know the person well, or something like Hi! In a reply, it is fine to leave off any salutation and just write, "Steve, Thanks for the invitation, but I just can't make lunch next Friday," or some such.
The main point is, customarily, "Dear ..." is a formal salutation. If you want something more intimate, it would be well to use a different word.