Thanks for the posts, this clears up my questions as well.
I think that sometimes people forget that different people come from different backgrounds. Two cruises ago, my wife and I sat with a couple at dinner from Texas, who never showed up to the formal nights. They despised wearing "purdy clothes" (as one of them actually called it) so they decided not to go, which I believe to be the wise thing. When in rome...(and if you can't do as the Romans do, go back to Greece for a short time!). I personally enjoy and am most comfortable in nice jeans, a collared shirt, and sports coat with some dress shoes during formal night, which is what I always wore to Carnival's formal nights. I do own a tux and a couple of suits, but I've always believed that if I'm on a cruise, and am in my late twenties, I don't want to have to dress up like I do for meetings in the business world. This is vacation, people! Dress DOWN!
Anyway, I do agree with dressing at least semi-formal (meaning a sport coat and no shorts/flip flops/tennis shoes) during formal nights, but won't look down on anybody for not wearing a suite and tie.
(Last cruise, an elderly gentleman and his family, children and all, continued to stare at me during formal night because of the jeans I mentioned above. It's as if they were offended, like their steak could not be digested in the presence of such an atrocity. I simply smiled and continued eating.)