Okay, you've been warned.
If you are going to wait the 10 minutes or so it seems for the Avengers : Age of Ultron end credits scene, then all you need to know right now is : don't bother. There is no end credits scene. There is only one line of text. And it says "The Avengers will return". Of course, we already know that, since we're going to see more action in Captain America : Civil War in 2016 and after that Avengers : Infinity War Part 1 in 2018 and then Part 2 in 2019.
On the other hand, the mid-credits scene is more interesting and worth staying about another minute or so for. There is Thanos (the guy whom we saw earlier in the rocket chair), putting on the Infinity Glove and saying "Fine, I'll do it myself". But the Infinity Glove is empty, so we know that he will be on the hunt for all 6 of the Infinity Stones to complete the collection, which will most likely be the topic of the Avengers : Infinity War movies.
Questions remain, however : wasn't the Infinity Glove supposed to be in Odin's own heavily-guarded Weapons Vault back in Asgard? But then, we've seen that it definitely could be breached. Or, like some others have discussed, perhaps Loki, posing as Odin himself, somehow gave it to him. Perhaps as part of a deal. Or a trick of some sort.
And there's the part about the Mind Stone being stuck in the Vision's head, how would Thanos get that one?
Well, we shall see, won't we?