Since a few people have voiced a similar thought to ^this, I wanted to address it. I mean, why did I use the word 'protagonist' and not just 'character'...? There's not much difference...a protagonist should be a character like any other.
But upon further thought, I think there is a difference and should be a difference, at least within the context of their own story. That is to say, I think a strong protagonist shouldn't just be one of many characters; there should be a specific reason why you choose to have the audience follow them, a reason why they are the best and most impactful POV for the world you're exploring.
Like, I think of the stories I write with large ensemble casts, with characters of many different backgrounds and ability levels...there's always a specific perspective or quality that sets the MC apart. They are driven to connect with their peers in a way the other characters aren't, or they have an intense hatred or sadness or other deep emotion that the other characters haven't experienced.
Or maybe they grow with the setting in a symbolic way, or they exhibit the worst or best that setting has to offer, so that the reader can examine both at once. Or, y'know, the way their mind works and the decisions they tend to make provide the reader with the specific understanding of the setting that I want to emphasize.
I hope this doesn't sound too pretentious, but I think that's the difference between writing a sequence of events happening to a person, and writing a * story*-- an effective protagonist should feel tailor-made for the story they're in, and vice-versa. It shouldn't be enough for the narrative to simply follow them or spotlight them, it should be commenting on them and interacting with them.
Like, if there's no real difference between them and anyone else in the story...then that's tantamount to saying there's no real reason for them to be the protagonist at all.