We sailed on QM2 last year and thought she was fantastic! The evenings do have a dress code as is Cunards tradition, they will either be formal (tux or similar for men, formal dress, anything glitzy usually does!, for women), informal (jacket and tie for men, suit or skirt and nice top for women) and casual (chinos or smart pants for men, skirt or trousers and top for women) The dress code is after 6pm and you are notified of the following days dress code in the daily news delivered to your cabin in the evening.
We found everybody did stick to this and it was part of the experience and we certainly enjoyed dressing for dinner.
On a transatlantic you will typically have 1 formal, 2 informal and 3 casual, although don't quote me on this exactly.
We like to do different lines to see the contrasts, but we will definately sail on her again, she has by far been our favourite ship.
Hope this is of some help...