Years ago, if a shooter was functional, it was interesting because it was a new thing. Shooters which innovated, naturally rose up to the top. Now, being functional and hitting all the checkmarks simply isn't enough. The novelty is gone and people want more.
Why shoot all these people? Well, because it's fun. Yeah, it is fun. Nothing wrong with playing games just for the fun of dismembering foes. But as more games come out, the standards rise, and we have the priviledge of playing games that are more than just fun, but really original or that otherwise have an impact on us.
In the end, in a gen where there were literally hundreds of shooters, it's getting a bit stale. And there needs to be a reward for the player to shoot enemies, or a reason to. Some games have nailed it, others haven't. Does the shooting have a purpose? Does it contribute to anything?
For example, non-linear combat options work. But one type of reward system that is pretty weak in my opinion is just a story. Shoot 10 people, watch a cutscene, shoot 10 people, watch another cutscene. There are still some GREAT games that have this reward mechanic (aka Bioshock Infinite), but shooting seems too often generically placed into a good story, which feels like a 3 hour movie, bloated to 10 hours with shootouts. This is unfortunately what I felt Mass Effect to be. A grand idea trapped in the confines of the modern action game.
A game I really like is Borderlands 2 because it's extremely honest about what it is. And knowing what it is, it tries in many ways to provide immediate and tangible rewards for the player's actions. For example, leveling up, gaining new weapons, being able to try the new weapons, and the appeal of having friends along to help you conquer powerful foes.