What was your rating? I might bump it up to an 8.0 .... I just need to see it again to clear up some things.
I don't necessarily mind Volstagg and Sif being so out of place ..... it's that the execution compared to previous credit stingers is slightly jarring. I'd go so far as to say in some ways the Asgardians looked more fish out of water in Tivan's lab than Thor does on Earth. In IM2 and Avengers they alluded to characters/situations you eventually see in the mid-credit. There is no mention in this film of Tanaleer Tivan even existing, nor do you ever get the sense that the Asgardians needed to separate the stones. It's just BAM, here's quirky guy and two of the lesser important characters from the film quickly dumping off an artifact that can destroy the universe.
I think they would've been better served showing some kind of discussion taking place about what to do with the stone, show them being teleported thru the bifrost, and THEN show us the interaction with Tanaleer.