I first learned game design from many hours of playing a whole panorama of manual wargames and then experimenting with rules mods. I learned how to do this from reading Moves magazine and from discussions with designers and testers. Very helpful and it helped me cut my design teeth for later development. I also got to see both good and bad examples of game design and how well-designed games are often the most elegant and playable. The more complex ones are simply unfinished designs, with paralyzing complexity and detail. This shows the designer didn't understand the essentials and threw everything possible into the game to cover all bases. I've also chuckled at pseudogrognards who can't see past this extraneous complexity and equate it with realism.
Once I felt comfortable with modifying rules and creating my own scenarios (again from Moves magazine as well as Strategy & Tactics magazine), I tried my hand at redesigning an existing game that had some warts and flaws. The motivation to do this was that I bought a game (Airwar) I'd eagerly waited several months to get that purported to be the ultimate game on air combat; instead it was instead an unplayable Byzantine abomination of complexity and inaccuracy. I said "I can do better than that" and accepted the challenge. JD Webster began doing something similar, only he tried to fix Airwar and instead developed Air Superiority. I found Air Superiority almost as hard to play as Airwar and knew my air intercept missile models were more accurate than JD's (I got mine from Fighter Weapon School instructor unclassified rules of thumb).
I tried to "avoid throwing the baby out with the bathwater" on the right level of playability and complexity for the subject, yet fix the unrealistic and inaccurate aspects of another existing playable but inaccurate game. That's how I achieved my first design: Flight Leader (AH) and Check Six! (USAF). I essentially redesigned SPI's Foxbat & Phantom, which didn't have accurate energy maneuverability and aerodynamic physics functionality.
After I'd gone through F-4 Phantom flight training, with all the air combat maneuvering and air combat tactics academics, and I was flying as a mission-ready fighter pilot, I was intimately familiar with the essentials of aerodynamics and physics involved in air combat. I now saw what was wrong with Foxbat & Phantom and clearly saw how to fix it. The designer of Foxbat & Phantom didn't have my advantage back in the early '70s. Air Combat Maneuvering is a complex endeavor and very hard to comprehend, as Robert Shaw later portrayed to laymen in his Fighter Combat book.
As Jim Dunnigan says, concentrate on the essential basic factors that directly determine the outcomes and succinctly characterize the activity. Nothing more and nothing less. Having successful game designs selling well on the commercial market is probably the most eloquent testimonial to credibility--John Warden found that a key endorsement when he hired me on the faculty at Air Command and Staff College.
Once I had redesigned an existing game, I was ready to do something more creative and original. It was obvious that no game existed that showed how an integrated tactical air defense system and tactical airpower overlayed and interacted with the frontal ground war at the tactical/operational level. Smart designers find a niche, need, and market and design to fulfill it. Using Jim Dunnigan's eclectic design principles (don't design something from scratch when something proven already works--use it), I took a pretty solid ground warfare game (Modern Battles, which I really enjoyed) and designed the Integrated Air Defense System, tactical airpower, ground C3, and ground logistics component overlays to the game. My new design was published by the Air Force as FEBA and later by Avalon Hill as Tac Air, which won a Charles S. Roberts award. I was then teaching threat systems and countermeasures academics as a platform instructor, plus I was pretty current and literate on modern USAF tactical employment doctrine. I was still wargaming some at that time and was just starting to get into computers.
In short, a guy who doesn't have one foot firmly planted as an operational warrior and the other as a serious wargamer doesn't have the advantage as the few lucky game designers who've enjoyed that luxury. As someone else noted, it also helps to see both bad examples and good ones so you can discern success from failure. I enjoyed better success because I didn't try to design a game totally from scratch right off the bat. It's always easier to stand on the shoulders of giants when attempting to rise to any new endeavor. If everyone tried to redesign the wheel, we'd never make the kind of revolutionary progress we now enjoy. I also didn't have such a big ego that I didn't realize my limitations and I never tried to do everything myself: I seriously sought feedback and incorporated other people's wisdom whenever appropriate. I can understand the problems military developers have when they have to deal with someone who outranks them and has deluded themselves into thinking they have all the answers--that will pretty much torpedo any team effort.
One of the many side operations StrategyPage runs is the Milgames listserv. This is for the people who develop commercial and Department of Defense wargames. Some of the comments there stand as statements worth passing on to a general audience. The following was written by Gary ("Mo") Morgan, LTC, USAF, retired. This item was part of a discussion of how so many Department of Defense wargame projects get screwed up.