So, let's start with the basics: What is a game designer?
A CommunicatorThe primary job of a designer is communication. This means you need to get used to doing a lot of writing, meeting, and explaining. Really, your ideas are actually secondary to the main focus of explaining those ideas. A designer with mediocre ideas and great communication skills is better than a designer with super ideas and no communication skills.
Why is communication so important? If the designer cannot communicate their ideas, then those ideas are stuck in his or her head. Without good design, it is harder for implementers to coordinate and plan the project. The designer should also be able to identify details that the implementers might not consider.
Not Really the Idea PersonSo, what about ideas? Most aspiring game designers are disappointed to learn that they won't get to be the "idea person" on a project. In fact, the direction of game project is usually already established by someone (maybe a senior designer), approved by executives (sometimes approving his or her own flawed idea), fleshed out in high level meetings, then passed down from the lead designer to a junior designer. Note that you will still have a chance to be creative, but you won't necessarily be suggesting sweeping changes and directly choosing which games will be made. You will probably be working on a smaller portion of the whole project. This is still an interesting job, however, and you should take the opportunity to learn as much as you can while doing your work.
Also note that most new designers are going to be working on the "second tier" type projects. These games include ports, sequels, and other games that aren't going to be as interesting to work on as the hot new high-profile brand. But, you have to start somewhere. As you gain experience and work into higher positions, you will generally be able to work on higher profile games and get more creative freedom, but will not necessarily call the creative shots. Even a lead designer will see many of the details about the core gameplay already decided for them. So, what does a designer do if he or she is not the "idea guy"?
An OrganizerOne major responsibility of a designer, particularly a lead designer, is organization. A lead designer organizes the different thoughts that everyone has come up with prior to starting on the project. He or she has to consider how these different systems work together and organize them into a series of documents. The lead designer also evaluates all the ideas and offer feedback based on my experience, particularly the parts dealing with online-specific aspects. The lead designer will also organize documents for the Creative Director, Executive Producer, and other designers to review and work on. After all this organization, the lead designer will still have to work on the details of some of the documents themselves along with the other designers. Eventually the design documents will be handed to the implementers to make early versions and prototypes of the game.
Good organization makes it easier to collaborate on the design. For example, there may be the design for a general advancement system in place in addition to a general quest system. A designer should be able to point out that some concept prevents them from working together, or that the advancement system expects players to gain X experience per hour, but that the quest system states it will provide Y per hour. Good organization skills to understand the systems are what make this feedback possible.
A ResearcherWhen it comes to the creative part of the job, a designer has to evaluate and flesh out ideas already presented. If the game contains RPG-style combat, for example, a designer needs to evaluate ideas for viability. If the ideas are not feasible, he or she must bring up these issues to managers and perhaps even suggest a way to alter the idea to make it possible. Once the idea is determined feasible, the design team needs to fill in details. How do you do this? Research!
One of the most useful skills a designer can have is knowing how to find information. In this day and age, that means knowing how to use search engines, online encyclopedias, and wikis; of course, you need enough basic knowledge to know how accurate the information you find really is. However, don't be afraid to go to the old-fashioned physical library in your town as well.
It also means having an extensive knowledge of other games, and that means playing lots of games! You should play major games in the genre as well as a few offbeat ones and a good selection outside your project's genre. Having a grasp on other genres can help give you inspiration for design problems you will encounter. Just be careful not to fall into the trap of trying to copy existing systems wholesale without understanding the design choices that went into that work.
If you don't enjoy reading pages and pages information, synthesizing data to present to other people, and playing games, do not become a game designer.
A Jack-of-all-tradesOkay, so now we know in general what a designer should do, what does a designer do on a day to day basis? Well, this is harder to define because it depends on many factors, including the project, the designer, and the organization. Unfortunately, the industry really doesn't have standardized titles or job descriptions. So, you get a lot of specialized titles that may mean different things at different companies. The skills required for a "systems designer" (which is my strength) are very focused on math. A level layout designer (which I am hopelessly terrible at), on the other hand, should be much more visually-orientated and able to visualize things easily in his or her head. Note that some companies might expect a "designer" to encompass many different disciplines.
It's not unusually to think of a designer as a jack-of-all-trades, especially as many designers have worked into a design position from another discipline. A designer with an art background might be assigned to develop the user interface, or work closely with the Art Lead to develop color schemes. Designers with a mind for programming might dig into the scripting system and help with implementation. However, a truly great designer should have at least a working knowledge of every aspect of game development. Even if the designer can't draw detailed pictures or make a 3D model, knowing how these tasks are done can help them communicate their ideas. Many designers also come form the QA department; this can be very beneficial if the individual had exposure to multiple elements of the game. And, knowing how to write test plans can help make sure that a design can be adequately tested.
In addition, knowing the different areas of game development helps keep the project possible. I've heard numerous stories about designers with no programming ability making designs that cannot possibly be implemented within a reasonable amount of time. Aspiring designers should get involved in multiple aspects of game development in order to make sure their designs are actually possible.
Putting it all togetherIf you want to be a great designer, you will need a variety of skills. This is one of the reasons why it's so hard to really write an exact description of what a designer is, and why it can be very difficult to teach game design in a traditional structured setting of university classes. On the other hand, it's hard to have the resources to train someone on the job. Which is why it's important to be self-motivated to learn all you can about game design, because it can be hard if not impossible to learn any other way.
Brian Green, known by the pseudonym Psychochild, is an experienced MMO developer. He's the former developer of the classic online MMO Meridian 59 and a developer for Storybricks. More of his writing can be found on his professional blog.