I always say g'day. Domestic that'll normally do. Depending on the reply if its an international flight I usually ask some courtesy question about their travel. Any conversation past g'day is always dependent on the vibe of the person.

Sat next to a girl (about my age, rather attractive) about 9 months ago, had the quick words. My seat IFE wouldn't work, I had a mag and a book but I noticed out the corner of my eye she was reading somethign which suggested she had the same work role as me. Asked her, she said yes, and we talked for the next 7 hours. Part of the story was she was moving to my hometown a couple of months after that flight, and we're great friends now...... all because my IFE wouldn't work.