I think some folks went adrift with the "hay-is-for-horses" bit and got away from answering the question.  My opinion (and therefore, only my own personal reaction to usage of the word "hey,") is as follows:

As a very informal greeting, I am not offended at all by someone using "Hey" for "Hello."  Note:  I would not use the term unless the person to whom it was directed was an actual "buddy," i.e., I would not use it with the local baker or dry-cleaning person, my mother-in-law, a professor, my doctor, or someone I just met, etc...you get the idea. But with my spouse, my good neighbor, my room-mate, my son, as a greeting, yes, I think it is actually a pleasant, very familiar way to greet someone you are quite close to. And bottom line: It's the tone of voice which tells the intent.

Another way in which I am very comfortable with the use of "Hey" is to interject it, for example, when excitedly introducing a (usually) positive opinion or observation about someone, OR even a negative opinion, but NOT ABOUT THE INDIVIDUAL YOU ARE SPEAKING WITH, e.g., "Hey, that's a beautiful jacket you are wearing!" or "So you think you goofed, but hey, we are all human!" And, "Hey! Did you hear they RAISED THE COST OF STAMPS AGAIN! (The latter is obviously not a slam against the person you are speaking with, but rather, a complaint or slam against "a system."

HOWEVER, past substituting "Hey" for "Hello" in a greeting with a close friend, and the attention-getting soft remark of introducing a nice thought or opinion, as described above, there is one other use of the word which I feel is EXTREMELY RUDE! This is the use of "Hey" TO GET SOMEONE'S ATTENTION, USUALLY TO REPRIMAND OR SCOLD THEM OR TO COMMAND OR DEMAND SOMETHING OF THEM.  Examples:  "Hey! Turn down the T.V.!" or "Hey! Look at me!  I am talking to you!" Or, "Hey! I'm over here (dummy)!"  Or perhaps, "Hey!  That's my pencil you picked up!"  I take that particular usage of "Hey" to mean I am being "talked down to" or reprimanded--again, it's all in the TONE OF VOICE, and is the equivalent of someone rudely grabbing my arm and jerking me around to face them.  Not nice, at all!   And it actually makes the person who uses the word in that manner appear to be controlling, rude, and disrespectful...sort of like he/she never learned good manners or to respect others' feelings.