What it is: Valve's new marketing buzzword for a new gamed called The Crossing that meshes simultaneous multiplayer action with a single player story campaign. Unlike most multiplayer games, crossplayer promises to put every player in a simultaneous protagonist role while other real players somehow co-exist in the same universe ... all orchestrated into a single story. Ambitious.
Why you should care: Valve made Half-Life, Counter-Strike and Team Fortress, all which closely rival sliced bread's greatness. Simply put, Valve makes great video games. When they speak, panties drop.
Why you should be skeptical: We've heard this kind of stuff before -- a perfect example was the marketing behind Shenmue on the Dreamcast. The promise was to morph RPG into something called "Full Reactive Eyes Entertainment." By Sega's definition, F.R.E.E. means that you can "Go wherever you want to go, see whatever you want to see, investigate whatever you want to investigate." Back in reality, Shenmue was one of the most linear games we've ever played with a lot of redundant dialogue inbetween. It was a cool game (who doesn't like sailors?) but we all know that it was more of a cautionary tale than an influence to other studios.
That said, it's hard to tell whether or not Valve really has something new that deserves to be called anything but a multiplayer game or an FPS MMO. I don't doubt that the game will be great, but whether or not their marketing people need a spanking for coining "crossplayer" is unseen.