I think looking into the original Halo game, and then FPS games in the years following may give you the information you're looking for.
Halo was released in 2001. Previous FPS franchises like Doom, Duke Nukem, and Quake, lacked much of a plot and were just mindless shooters. Rainbow Six, Metal of Honor, and Battlefield began the trek towards "realistic shooters". Half Life, and especially Half Life 2 started exploring actually having a good storyline. Throw in Counterstrike and you have a lot of very good games, but they all have very similar mechanics.
Amid all of this, Halo was made, with regenerating shields, well balanced multiplayer, an interesting story, as well as unique vehicles and weapons. There were also a lot of smaller additions that were unique to Halo at the time, like the ability to throw grenades and melee without putting away your weapon, making combat much more interesting. Within a year, just about every FPS game had at least some of these features.
Copying what works is a safe way to guarantee sales. Its not even as bad as you might think, there are any number of ways to mix concepts, allowing developers to make unique games without having any truly unique elements.
But the FPS market has been largely dominated by sequel after sequel ever since Halo came out. With lesser sales going to blatant Halo clones. I expect the same to happen with Brink if it becomes as popular as I think it will.