My totally unscientific theory seems to be that the more repeat cruisers, the more formalwear. Also, the longer the cruise, the more formal (which is weird because I'd think people would want to pack less).
Also, while there are always people who run the gamut of formal to casual, if you're in the atrium before dinner on formal night, it's going to SEEM like everyone is dressed formally (because they're all getting pics taken).
That said, I've never worn anything more fancy than a summery dress. That's formal for me because I'm not in sweats.