Audiences have come to expect a mid- and post-credits sequence from Marvel Studios. They've come to expect them to the point where Joss Whedon felt compelled to warn people his last film for the studios, Avengers: Age of Ultron, wouldn't have a post-credits stinger just so audiences wouldn't get mad about sitting through credits for nothing. Some people hate the credits stingers, complaining about... well, I'm never quite sure what the argument is, as their placement after the events of the movie make them extras, things you can easily skip without missing much.
They haven't all been great - I still don't understand why the Norse gods brought the Aether to The Collector at the end of Thor: The Dark World - but some have been sublime, like the now-famous shawarma scene in The Avengers or the Howard the Duck bit in Guardians of the Galaxy. But it wasn't until Avengers: Age of Ultron that Marvel served up a credits sting that was actually bad. And man, this one was bad.
Once again we were treated to Thanos, whose appearance at the end of The Avengers was actually awesome. There, as a guy in make-up, he said nothing, but he smiled when told that fucking with the Earth was courting death. For newbies it was mysterious and odd, sending many people online to find out more about this weird character. For comic readers it was a nice nod to Thanos' story - a demigod who is in love with the anthropomorphic embodiment of Death, he seeks her approval by wiping out half the galaxy.
But this time Thanos' appearance wasn't mysterious. He had already shown up in Guardians of the Galaxy, and we had already heard Josh Brolin voice him. And it wasn't odd. He opened up a cabinet, pulled out a glove (the Infinity Gauntlet, for the comic readers) and said a quick line of dialogue. To make matters worse, his appearance didn't even feel like Thanos for comic readers. His pithy line - "Fine, I'll do it myself" - doesn't have the grand eloquence of Thanos' over-the-top comic book speechifying. It doesn't have that heightened, pulpy, faux-Shakespearian edge that Marvel Comics used to make its cosmic stories feel grand and smart, even when they were silly. It was just a dumb joke. And if we're really going to get into nerdy issues with it, the Infinity Gauntlet was already seen in Odin's secret stash in Thor... although that still makes no sense, because in the comics the Infinity Gauntlet is just a glove. It's not a special device, it's not a magical item, it's just Thanos' left glove, which is where he opted to mount all of his Infinity Gems. It could have been the Infinity Tiara or the Infinity Medallion or the Infinity Choker. There's literally nothing special about the glove, so why the hell is Thanos keeping it in a weird cabinet?
Worst of all, though, is the fact that this mid-credit sting isn't teasing the future or showing off something fun about the characters or the Marvel Cinematic Universe - it's just responding to fan complaints that Thanos has yet to acquire any of the Infinity Stones. Yes, it's weird that over the course of all these years none of the Stones have made their way into Thanos' possession, but I would rather see that be explained away in-story than lampshaded with a dopey joke. I would rather find out that while Thanos doesn't have the Stones himself he has been manipulating things to get the Stones where they are so that he can come collect them, as hoarding them to himself would rouse the suspicions of bigger entities in the universe. Making the current placement of the Stones part of his plan would also answer the question of why he was so stupid as to give one away to Loki back in Avengers. That's the kind of complex comic book writing I like, the kind that takes flaws and mistakes and weird story dead ends and turns them into something fun and imaginative, not a cheap handwave aimed in the direction of a bad parody Twitter account.
The contents of these credits tags don't really matter - at least one has already been retconned, and others don't exactly make sense - but they're a lot of fun, and they've become one of the defining elements of the Marvel Cinematic Universe. Walking out of the theater buzzing not just about the movie but the fun tag is a big part of the experience. I wish Marvel had spent just a couple more minutes on this one, even simply giving Thanos a legitimate speech, a good cosmic villain monologue, in an attempt to capture the character.