Unique Selling Points, USPs (or sometimes called Key Selling Points) are the elements that make your game stand out, what makes it better than and/or different than titles from the competition. Note the "Selling" part. These points are supposed to make the players want to buy the game. Or at the very least, make the publisher want to invest in that project :)
A USP isn't necessarily a game mechanic. It can also be a specific visual style, an original location or story, or even a tremendous amount of content. A few examples:
- Jet Set Radio: Manga-like visual style
- Fight Night: Innovative punch controls
- Dead Space: Monster dismemberment
- Metal Gear Solid 3: Survival and camouflage
- Vietcong: Fight in the jungle during the Vietnam War
- Blinx the Time Sweeper: Manipulate Time in various ways.
- Afro Samuraï: The ability to slice the enemies.
Note that obviously, I've only chosen only one USP for each game, they have many others. There is no rule as to the number of USPs a game should have. I'd say most AAA games have 3 to 5 USPs. If you have too few USPs, the game might not feel rich enough, or might not be able to really stand out. If you have too many, you won't be able to focus and prioritize the development properly.
As stated at the beginning of this article, a USP is first and foremost an argument to convince potential buyers. As a result, you usually only have to look at the game's back cover (or Steam or whatever digital distributor) to find the game's USPs.
As with everything you decide during the game's development, your USPs must comply with the mission statement.
Finding a USP for a project is usually easy, since it basically describes the main ideas you got for the game. Finding a catchy way to phrase them is another story.