Let's not forget Fonzie's "Heeeey" greeting, which was supposedly a bit of horse-play on Latino machismo, Fonzie was Fonzerelli, an Italian, and the big joke everybody was laughing at was overly-macho Latinos, and his Heeey was said with the cheeky flair of a "Hey, now there's a real man in your presence kids"
But it is true that Hey was traditionally an angry or excited
shout, like "Hey you boy" or "Hey, let me out" or "Hey look at this". The dictionaries say something like
"Hey" is traditionally an interjection used to draw attention. I think, as Bobsy said, it really depends on how it's said. I might say (with a smile) "Hey, look at you Mum, you look like a model today". And I think what young people today are doing with their single Hey is leaving out the rest, in other words they're really saying "Hey, it's great to see you" or "Hey it's the one and only .." so there are sometimes sarcastic undertones with Hey used in this way as a greeting.