On such a big ship, there will be lots of people in your situation. Some people go to town on formal nights, others dont. As long as you're smart (ie not jeans, shorts etc..) you will be fine. Some women where trouser suites or just nice dresses etc... this doesnt need to be a stress. If you want to miss the formal nights all together, you can still dine in other restaurants / buffet of which there isnt a dress code.
Just returned from a med cruise on Navigator (28/7/07). On the formal evenings ladies generally wore long evening dresses or shorter cocktail dresses. There were also a few ladies in trousers with glitzy tops. Most people on the cruise dressed up on the formal nights. I have now completed 3 cruises so pretty much a novice compared to some, but on my first cruise I went to a large department store at the end of the sale and picked up a couple of classic evening dresses dramatically reduced that wont go out of fashion, and have taken them on all subsequent cruises. Like yourself I am not a dress person but I did want to fit in. The dresses have served me well, and I dont feel that I am not fitting in with everyone else. When you see everyone else dressed up you will be pleased you have done the same, there is a really nice buzz when you see everyone else.
You'll see everything from ballgowns to smart summer dresses. I tend to stick to black pants with glitzy top.
Wore short cocktail type dresses, most from my children's weddings over the last few years, husband did buy a dinner suit for this cruise and we both enjoyed the dressing up bit.
You can wear any smart type of clothing trousers and evening type tops.