Holland America ships are reserved in their decor. They don't do 'wow', instead HAL leans towards the more classic cruise experience. HAL is a premium cruise line, meaning more upscale although the prices often don't indicate it. The typical guest is likely older and more well-to-do than those you'd see on the mainstream cruise lines. HAL most closely resembles Celebrity to me.
The Nieuw Amsterdam is a beautiful ship and actually considered a bit small in today's market of mega-ships.