Not much. I hear it as a simple variation to break the monotony. When two good friends meet, "Hi" seems to work fine, because they have plenty to talk about. When two casual acquaintances meet, they're often searching for something clever to say, to avoid seeming stupid. "Hi there," makes the conversation last twice as long as, "Hi." If it's a fairly new acquaintance, "Hi there," or "Hi yourself," seems to indicate a desire to continue the conversation, where a simple "Hi" could be taken by the other person as dismissive, depending on the inflection.
Edit. "Hi yourself" is only used as a reply, or a returned greeting, while "Hi there" may be the initial greeting. (in case you didn't know)
Even from a complete stranger, "Hi there" would be taken as a more "forward" greeting than "Hi," suggesting "Hey, let's talk," again, depending on the inflection.