If it ain't broke, don't fix it. Most FPS are trying to break out of repetitiveness with different features (invariably the weapon selection, although these are often much more similar than you might think) and set-piece level or design. Although the premise is often the same, I still find a well-put-together FPS to be very satisfying. It's not just the gameplay, but the presentation, and when a game pulls both elements off, it can be a very enjoyable experience.