Or is the story simply a means to deliver Easter eggs, cameos, and bursts of nostalgia to a very specific audience steeped in the lore of the source material? When people use the criticism "fan service," they mean the latter.

There are plenty of great superhero movies, sci-fi movies, etc. that contain Easter eggs and so on, but the primary focus is to tell a great story. I just rewatched Spider-Man 2 the other day-- I forgot it has Curt Connors, John Jameson, even a Doctor Strange reference. But those are just passing things for fans that have no impact on the story being told-- the story and the filmmaking are what make that movie exceptional.

Whereas a number of movies these days primarily exist to create nostalgia-based pleasure, to reference old things fans might remember. (Say what you will about South Park, but that "member berries" season is one of the most on-the-money cultural commentaries I've seen from a TV show or movie in the last decade.) The Deadpool Wolverine movie is a prime example of this.