PlainBlueSky wrote:
I take so long recognizing people that they've usually passed by the time I realize who they are...
I'm the same way. I'll be walking down the street, and someone passing me will say, "Hi!" and I'll be like, "What?" That happened once with someone I hadn't seen for months. I sort of lifted my hand in an absent wave as I attempted to remember who she was, but by the time she'd passed me I'd remembered.
As for the acknowledging thing, I find it doesn't matter. It depends on the person, and on you. I generally don't acknowledge people I don't know when I'm at school because this could be seen as a bit "creepy," especially if the halls are crowded (o_o that's a lot of "hellos"), but I often acknowledge people when I'm walking down the street.
But just a quick guideline: Don't acknowledge someone in passing if they're on the phone, or talking to someone, or listening to music. I generally don't acknowledge people unless it looks like they'll acknowledge me back. Generally this type of person looks polite, generally in a good mood, and laid back. Usually they aren't wearing dark, bulky clothing or fur coats, and they don't have grumpy looks on their faces (avoid acknowledging these people -- usually they only nod curtly which can feel awkward)
Hope this helps! ^^