Some people who barely know me greet me with "how are you?". Now it's not that "how are you?" is a bad phrase, or a bad thing to say. Just that it's a filler for a situation that requires a sentence, without giving it any thought as to what you are saying. It's fine to say "how are you?" if you mean it. But otherwise it's pointless. And if you ask this person the same question "and you?", they won't say "oh today I'm excited cause I'm doing this" or "today I'm a little depressed cause I gotta do this". They just say "fine". So what's the point? Every time you ask the question, no matter how the person actually is, they will say "fine".
On the other hand, "what's up?" is just a little less pointless. The answer to "what's up?" is usually "oh nothing much", but there's a greater chance a person will tell you what they're doing than how they feel, because the latter is more personal. In fact, some people answer "what's up?" very sincerely, they will actually tell you.
I prefer 'sup', myself :D
Another one that's completely overused is "Take care" but Jerry already tackled that one for us.
Yeah I hate "how are you." That and "so tell me."