When explaining Dnd, which many have endeavored to do in order to reign in friends as players...you must start by saying it's a game, a game played with only paper and dice. The mechanics of the game all lie in the interface of PC and DM imaginations.
And if this is the case, why do we need rule-books?
With every released edition of Dnd the rules become more encompassing, more specific, and more stringent. This causes a huge gap in the playing styles of older generation players and the new.
Younger players are immersed in a world of technology, video games that provide instant gratification with a couple button clicks are competing with traditional fantasy mediums like novels and of course Dungeons and Dragons. Because of this new generation where the principles of creativity and complex problem-solving are not emphasized, gaming companies have created systems like 4.0 edition. The central focus of the rules is on combat, the system enables easy, non-committal play.
Personally, I know exactly what I want out of the game and it doesn't involve rapid-fire combat. I love creating intense stories that pull in character emotions and reactions. Building worlds that are culturally interesting with diverse and rich histories is like my DM crack. And most of the time the rules just don't fit the bill.
Rulebooks should simply be viewed as supplements, they do not control the game, the DM controls the game. The books are sources of ideas, full of interesting spells, items, and monsters. Using this mentality I create games using multiple systems, I read the AD&D PHB just as frequently as I read the 3.5 PHB. I don't like to stringently commit to one system because I use pieces of many, and often just make up things based on the way I want to do them.
The emphasis on role-playing and story-building in the classic editions gives a gaming structure that speaks to the heart of what Dnd is, but I find its options lacking so I go to the 3.5 books.
All in all the rules for all systems are simply guidelines for those who need them, and never a controlling force.