Glen, it seems like you're throwing around a lot of judgments based on little information not only from the person in question, but what you have provided us.
- Where was this greeting held? Was it busy? Were they engaged in conversation with someone else? Were they listening to an MP3 player? Was their attention otherwise engaged? Were they hard of hearing?
- Did they honestly hear you? There have been times where I'm in my own head thinking or lost in my own little world and someone has said hello to me and I didn't even recognize it. It happens. Just say hello again if the situation permits.
- Was the greeting in passing? What was their body language? Their mood? They could've been angry and would've snapped at you? Would you prefer that? Maybe they are depressed?
Please provide people with the benefit of the doubt before making judgments. Furthermore, how can you judge "volumes" about their ENTIRE personality based on whether they say hello or not? That's a pretty big label to just arbitrarily slam on someone for what could have been an honest mistake.
I do agree with you that it's impolite to not greet someone back when they say hello but if they don't reply then I just shrug it off and not let it bug me. If it happens again, then I tend to question.