Among the unending horde of teasers, trailers, and ever-annoying parody advertisements from GameStop, demos stand out as a unique way to promote a video game. Instead of flaunting an image or video specifically tailored to garner attention, game demos place a piece of the actual game in the player's hands, allowing them to get a feel for the style and flow before actually purchasing it. This should, in theory, dodge the bias inherent in marketing material, in that it is designed to be marketed. Unfortunately, it's not a bullet-proof system.
A game demo is equivalent to an excerpt taken from any text: It's a brief portion taken from a larger work that isn't necessarily representative of the final or complete product. However, this effect is minimized by the interactivity of gaming because players do more than simply look at gameplay; they experience it firsthand. Regardless, demos cannot be taken as an indicator for the entire game, largely because the remaining portion could easily be worse or entirely different.
A timeless example of misleading demos comes from Aliens: Colonial Marines, the botched FPS project that came out of GearBox, developer of the critically acclaimed Borderlands series. Before crashing headlong into a wall of negative reviews (and only after an outsource-ridden development cycle), Colonial Marines showed off a brief press demo. Not only was the demo level created outside the domain of the core project, but it never made its way into the actual game. As such, it showed a product that was in no way related to the absolute nightmare that the game proved to be. However, that demo is also the only reason the game sold more than a handful of copies.