I think Iron Man 3 threw the pattern out the window and that was responding to Avengers one foreshadowing one joke (they skipped the foreshadowing in Iron Man 3 and just had the joke). Then there's Captain America with one foreshadowing and no joke. GOTG was going to be no foreshadowing and two jokes, but ended up with just the one scene (like IM3). I think the point is there's no pattern anymore. The only rule is they'll at least have one scene (except The Incredible Hulk, which obviously had no post-credit scenes).
Didn't it have that scene with Tony Stark? Or did that end up before the credits? (It's certainly in the vein of the post-credit scenes in all of the others.)
It was before the credits. It probably should have been after, but, for some reason, it wasn't. Maybe they didn't trust people to stick around just yet, so they decided not to risk it.
The director, Louis Leterrier, doesn't care for the idea of post-credit scenes, so he put the stinger at the end of the movie proper. This was, of course, when Marvel Studios was new and still figuring out their game-plan. I'd bet that if the movie were made today, or if Kevin Feige knew then what he knows now, the scene would be post-credits.