--Warren Robinett
Which isn't to suggest that simplicity is bad. Only stupidity is bad, which I might define as thoughtless simplification. Artificial simplicity is necessary for any act of interpretation. Or any kind of representation. The world may be everything that is the case, but it is infinitely complex. People who describe and interpret things need to choose the relevant details (what Martin below would call salience) and represent them in a way that clarifies our choices. There are a million examples of this fact (Tufte's books are full of them) but my favorite recent one is in this incredible game-design textbook called The Rules of Play by Katie Salen and Eric Zimmerman. It’s an innovative and inspirational textbook for a number reasons, but I’ll save that for later. For now, check out their implicit critique of overly complex simulations, which is itself prescient of Wii’s victory in the next-gen console battle:
“Why is it that games can’t simulate everything with a high degree of detail? Why can’t a game simulation be both wide and deep? ….Limited development resources require that game designers decide where those resources will be spent. But the limitations of time and budget are not the only things affecting the scope of simulations. Meaningful play results from the ability of players to make meaningful choices from a limited set of knowable options.”

Zimmerman and Salen take the point further as they outline an approach for designing wargame maps. Citing another game designer, James Dunnigan, they describe how careful formal choices (thoughtful acts of simplification) create the possibilities of meaningful action. Including details that don’t impact game play are merely distracting.
“As Dunningan puts it, too much detail in the terrain can get in the way of a player’s understanding; only “gross” terrain features have a real impact on military operations. Abstraction emphasizes the features critical to understanding the terrain, while minimizing the “noise” created by less important elements…. A visible feature that does not contribute to the functioning rules of play is bad design.
Including details that don't matter makes bad briefs, and bad marketing. Simplification is a great end, but you can’t start there. You are just as likely to pick a meaningless as a meaningful detail. You first have to analyze the data to identify what matters—stuff that impacts the relevant outcome. People who demand or, worse, assert simplicity are usually just being stupid or trying to make you feel that way.