Many of the older ships of the modern cruise lines are relegated to less popular itineraries. Many quickly lose their luster on these runs. Top notch service and entertainment staff go to the new ships, many times. There is a slight tendency for old ships to be less clean in places where you want clean - the kitchen, the bathrooms. Cruises on older ships tend to be cheaper, especially when sold to an alternate line, renamed and stashed somewhere, like Aruba, for a home port.
New ships sparkle with new paint, carpet, equipment, kitchens, showrooms, etc.
The best of the best cruise staff, entertainers, wait staff and stewards tend to be sent to the new ships. They have better stabilization, guidance systems, etc. They get choice runs, many times. It is especially great to be on a new ship during its maiden voyage - though service may slip here and there, there are amenities and a special ambience that we love. New ships are at their best after six months or a year of breakdown cruises, when everyone is sharp and everything works.