As for Formal Night attire, you will see EVERYTHING that night! You will see tuxedos, gowns and tiaras and you will see shorts and tank tops.
Last year, since we had so much stuff with us, we decided to rent a tux for Rob and I wore two black dresses, one on formal night, the other on semi-formal night. I must say that this time more than any other, he stood out a little and we saw only a handful of tuxes. I would say that the majority of men were in slacks and a nice top.
I never saw any really dressy ladies this time, most with a cocktail-type dress and/or a pantsuit.
The ONLY people that didn't fit in and people were looking at were the ones that wore shorts and tee shirts/tank tops in the dining rooms.
To me, that is just rude. If you want to dress like that, no problem...go to the buffet, it's almost the same meal and you will fit in with the shorts/tees. If you are going to eat in the dining room, make some kind of an effort! You absolutely don't need a gown/tux/suit...but a nice pair of pants and a nice shirt/blouse will do just fine!
In 2005, my luggage didn't arrive until after 10pm and the only thing I had was shorts. Even though it was the first night and it was a casual attire that night, it just felt wrong to wear my shorts to dinner in Lumieres.

I felt so out of place. I LOVE shorts and could wear them everyday but it just felt wrong to be wearing them in a lovely restaurant. Just before dinner we ran into Mike & Robin in Mickey's Mates and they were SO gorgeous!!! I was SO embarrassed

They were all clean and purdy and I looked like I was ready to hit the Magic Kingdom
We won't be dressing up as much on this cruise (luggage restrictions! and a three-year old

) but I will make an effort
Ok, I will step down off my soapbox now