Yes, we've been reciting this mantra for decades. Designers, reviewers and players all believe it. However, we generally worship gameplay for a rather uninspiring reason: It's the "fun" part of the game, the part that hooks you. Here's a more interesting reason that gameplay is important: because it sets games apart from other mediums and makes them unique. If you're not there for the gameplay, why are you playing a game instead of watching a film or reading a book?
Unfortunately, gameplay rarely resonates with what a game is actually trying to express. It's not that gameplay is an afterthought in the design process. In fact, the exact opposite is usually true: Gameplay is the forethought, and everything else, including the developers' artistic vision, is slapped on top of it. Consider the standard list of game genres (FPS, stealth, platformer, RPG, RTS, adventure), and notice how it's really a list of game mechanics. This differs substantially from the way film is categorized, where genres are based on content and intended emotional effect.
Many modern, mainstream games deliver aesthetic experiences that might pass muster for Ebert, but because they're an afterthought to the gameplay, they're shoehorned in through cut scenes and linear storylines. The result is that the gameplay, the very heart of the game, is out of sync with the game's overall artistic expression.
BioShock presents a perfect example of this kind of dissonance. Through its non-gameplay elements (set design, audio diaries and linear story), the game successfully explores the shortcomings of an extreme Randian philosophy. The gameplay, on the other hand, involves upgrade-heavy first-person shooting.
If we're making a game that deals with individual freedom versus the good of society, as BioShock does, why not devise game mechanics that explore these issues directly? Why permit the artist's vision to manifest itself in the storyline and environment design but not in the gameplay? The answer, looking at BioShock's development history, is that the designers set out to make an FPS long before they figured out what they wanted to say with it. Thus, BioShock became yet another victim of the misguided "gameplay first" design philosophy.
To make games that are works of art, we should be taking the exact opposite approach. We should figure out what we want to express with our games and then devise game mechanics that best communicates that message. The heart of our games, the gameplay, should be our primary vehicle for expression.
Once we use gameplay to communicate more, we'll rely on tools borrowed from other media, such as cut scenes, less and less. We may someday look back on this era and chuckle - the old days of game design, back when we were still using cut scenes and linear storylines - just as we chuckle now over old movies that relied on title cards to communicate something as simple as "Later that afternoon ..."