The core purpose or result from game design is the creation of the games specific rules and content. For example, this would include the art style of which the game will consist of and will influence all future concepts, ideas, models etc. Although that is a very simple description, game design is the most crucial part in a games development as it can either make or break a game from the get go.
Despite this, game design is a very broad and complex process and although there are specific people employed in studios for this task, it does include many if not all the members of a particular studio on some form or another. the specific peoples involved in the design of the game primarily consist of a select few categories; World design, level design, user interface, content, system and the games writing. Each process is critical to the games final outcome and all must conform and work with each other and to the desired result from the game.
Gameplay is perhaps the first thing that comes to my mind when I think of game design, and this is because gameplay consists of so many elements. For example, the fundamentals of the game are down to gameplay, for example is the game an FPS, or a platformer? This would of been a decision at the very start of the process of the development of the game and without this simple yet crucial piece of information, the game could not be created. Gameplay however, primarily consists of how, when and where the player interacts with the game world, such as action sequences and the basic control layout etc.
Personally for me, I want my games to have solid interactivity with the game world, and to have a consistent level of quality throughout the game. Perhaps the most major thing I look for in a game is immersion because to me, immersion is what keeps the player playing the game and the easiest way to break this immersion is from a simple glitch or bug in the game. Despite the fact that its impossible to create a game without any faults, its the consistency throughout the game world that will keep the player interested and wanting to continue. Although I regularly come back to the uncharted games, these to me, are the best example of solid consistency and not giving the player a chance to put the control down. There are no loading screens and nothing in a sense that breaks the immersion for the player.