It’s hard to specify exactly what Gameplay is but it’s an element that is the most important in my opinion. The other elements are about Graphics, sounds but Gameplay is mainly how it all works together and it involves pre-dominantly the controls for the actual games, how the players interact with it. Its important because, well a good example that I can think of if you go to a foreign country in a certain car; the experience is changed by what car the people are actually in, an internal environment with external features if you will.
If seated in an old battered car with awful seating in Venice for example compared to a more open and comfortable vehicle the "value" is affected by the actual conditions. Basically for me like from this example the conditions that an area is seen in effects the actual area.
For a gaming example you could have a really intense environment with beautiful 3D models/sounds but it could be entirely destroyed by how the player operates in this environment.
Also the Heads Up display (or lack of it) graphical elements of the game can come into terms of game play as well.
Call of Duty for example, its a big success in many factors, 3d models sound etc but the big part of it is how "real" it feels; Looking down a gun cross-hair blurs the image slightly, when shot you have the intense red screen of blood, controls are in comfortable positions for moving the characters around. Gameplay is defiantly a design element to a game, if it was ignored then its just foolish in my opinion. It is an element to me that can enhance the game of make it worse, game play comes first to me and the other elements second.