I enjoyed The Avengers. I thought the first act was a little rough, and that the opening sequence suffered from some flat action choreography and some unfortunate exposition dumps at the expense of character, but overall I thought it was a solid, enjoyable superhero movie. Joss Whedon's banter and direction were the perfect match for the ensemble, and he's been the most successful director to date to find ways to motivate the Hulk and turn him into a believable monster.
I sat through the closing credits, too, knowing without being told that there would be post-film scenes. There were, in fact, two: one setting up a villain for the inevitable sequel, and another that called back to Iron Man's desire to get some shawarma. The shawarma scene was pitch-perfect: no dialogue, longer than expected, and a wonderful joke about how the battles in these movies always leave their cities far more ruined than we realize.
But it was the villain's tease that got me thinking about post-credits scenes in general, and the two main problems with their popularity. The first is that they're almost always worthless, doing nothing to enhance the narrative we just saw play out or set up the one to come. Every one of Marvel's recent Avengers movies has had one of these scenes tagged on after the credits. They're designed to build hype for future installments, so they feature characters from other movies in an effort to make the whole thing feel grander and more interconnected. Nick Fury visits Tony Stark; Tony Stark visits Bruce Banner; Nick Fury visits Captain America; etc., etc.
The tricky part, though, is that these scenes don't/can't feel like part of the "real" movie. The films are designed to be seen as a series, and they've all got their own stories to tell. You can watch them all in a row, skip the post-credit scenes, and be just fine. (Well, you'll have watched Thor, so you won't be fine, but you get the idea.) The scenes are like extremely high-quality fan fiction: all the characters are there, but the action is non-canonical.
The bigger problem, though, is that these post-credit stingers in modern action/superhero movies are training us to view the movies not as stories, or even pieces of entertainment, but as links in a chain of marketing materials that will never end. It's one thing for a film to set up a sequel or leave room for growth via action or dialogue, like the discovery of the Joker card at the close of Batman Begins. Those moments are cues that more will come, but they're also woven as naturally as possible into the film itself. (Though Corey Atad has written about how even these scenes can damage a film.) They are signs of a larger filmic universe, but they're also part of the actual film. When the credits roll, the movie is over. Whatever happens or doesn't, whatever sequels are made or aborted, you still have the movie in question. Another great example: the discovery of pilfered jetpack designs in the final moments of The Rocketeer. Disney left room for a sequel by giving their hero the ability keep flying and fighting crime, and even though a follow-up film was never made, that moment feels right — feels true — within the larger framework of the film.
The post-credit scenes favored by recent Marvel movies don't do that, though. They turn the film into a giant ad for its sequel. The point of Iron Man isn't that Tony Stark became a hero; it's that Iron Man was created so he could be contacted by S.H.I.E.L.D. via Nick Fury. The point of Thor was to introduce Loki, who would reappear in The Avengers. The point of Captain America was just to get the man to the 21st century, love life and consequences be damned. The movies exist only to point the way to what's next.
Some of these movies have been good. (Iron Man and Captain America were wonderful.) Yet none was allowed to stand on its own. Sequels and series are nothing new, but the audience — and the film — doesn't need a teaser to show the way. Popular characters always return. When a James Bond film ends, we know he'll be back; we don't need photographic proof. And whatever movie comes next certainly doesn't need its predecessors post-credits moments to act as weak connective tissue.
Can these moments be fun to watch? Yes. There's no getting around the excitement at seeing a surprise glimpse at what might come next. (Even if the scene is attached to a movie that underperforms, making its sequel plans much more uncertain.) But the buzz of those moments isn't enough to balance the fact that these scenes are leading us to focus on and care about the wrong things. We can get so wrapped up in what's going to happen later that we forget to look at what's happening now.