Additional clarification:
I do enjoy dressing up a bit (emphasis on "a bit") and would not wear shorts to dinner in any setting outside my own home, so that's not a concern. One of my cruise companions asked if he really had to wear a suit on formal night, because he doesn't have any suits that fit him currently. Sounds like the answer is a resounding "no," so that's helpful. It also sounds like a collared shirt and pants, or a nice sundress for ladies, on formal night is not outside the norm.
It may be an unpopular opinion, but I do think that what you choose to wear is a sign of respect to others, especially in a fine dining environment (say what you will about the MDR, but at its best, the intent is for it to feel like fine dining). I know that not everyone feels that way, so I'm not going to judge others' choices, but I personally want to be respectful to those that may care.