I'm one of those who like to dress up. On most cruises I take my tux, which I bought some of my wife's business functions before we even began cruising. On a couple of cruises I've worn a dress shirt with tie (with or without a sports coat), and on one cruise I wore a kilt with the jacket, tie, etc equivalent to a tux. DW brings a LBD that she wears.
For casual nights, I take two golf/polo shirts and two pairs of pants, which I alternate. Following that practice, I wear my tux for both formal and informal night so that I can justify the weight and space of taking it. One night I wear it with a cumberbund and the other night I wear it with a vest so that I get a slightly different look.
On our last cruise (7 nights in the Caribbean) I only saw 3 or 4 tuxes, and about 1-2 dozen suits, the rest of the men wore dress shirts (with or without a tie), polo shirts, and even a few aloha shirts. I've usually seen a few military uniforms but not on this particular cruise. As a general rule, the women tend to get more dressed up than the men, with LBD being most common, and children can be seen in almost anything, including princess gowns.
Bottom line: wear what you like (within good taste of course), but don't be afraid to get all gussied up if that is what you want to do. You won't be alone no matter what you do.