Recent greetings that I have heard that just stop me dead in my tracks because I have no idea how to respond:
You get a call from someone or you call someone and the first words out of their mouths are:
"Hey, whadda ya know?"
Dead stop. Train of thought instantly derails, conversation ball rolls under the couch. Is he asking me a question? Is he thinking out loud? What do I know? I know lots of things... how, what? WTF do I say to something like that?
Similar to this is
"Hey what have you been up to?"
And they're not really asking. This isn't an opening conversation gambit, it's just something said as a greeting. What have I been up to? Slam on the conversation brakes. Screeeeeeeeeeeeech. I've been up to a lot and nothing at all, WTF do I say to something like that?
Usually I respond "Nothing" which is really lame and again, the conversation dies to an uncomfortable pause.
I would accept both such greetings as rhetorical salutations, similar to "Hello, how are you?" As such, IMHO, a simple "Hello" or "Hi there" would suffice, by way of response. This is open to interpretation, of course, and one can choose to be more literal in one's response, should one choose to do so, with something like
"Hi. Not a lot! You?" When someone greets me with "Hello, how are you?" my standard response is "Fine thank you. How are you?" or simply "Fine thanks. You?" Even though I'm aware the initial greeting was rhetorical, I still respond as I do, as a matter of courtesy.

I've never seen someone struck speechless when asked "How are you?" when greeted such, but I have seen such a greeting open a real Pandora's box, whereby the greetee takes the greeting literally and proceeds to rattle off, ad nauseam, a lengthy diatribe of health problems.

I guess there are no fixed rules. I have a dear old friend who always greets me with, "Hi Kerry! What are you doing?" whenever he phones. It's his cute way of saying "Hello!" Because we've been friends for over 30 years, I always respond literally with "Washing up" or "Making the bed" or whatever it was I was doing when the phone rang. We laugh and get on with our conversation. I've recently taken to greeting him in the same way, when I phone him, with "What are you doing, George?"
