I usually nod and sort of smile hello, not wanting to appear to be one of those gabby old lady types. I think most people take that as an I-don't-want-to-talk message, but it's just shyness.

I've found in biz people are a more polite, and an initial greeting is pleasant, with a bit of chit chat during the meal.

It seems odd to be sharing an armrest with a stranger and not have a small bit of conversation during meal service. We each have a speaking relationship with the FA, but not each other.

For those of you who have forgotten, you can't hear anything above the engine noise in Y anyway, so conversation is limited.

Or shouted as one traveler did all the way from SFO-BKK to a poor stranger across the aisle. He NEVER shut up, and the other guy only mumbled um, uh, for 8-10 hours or however long that flight is.