Fans and Moffat have two entirely different meanings for the words "Anniversary Special."
For Moffat, an "Anniversary Special" is "a special that airs on the 50th anniversary."
For fans, an "Anniversary Special" is "a special that celebrates the 50th anniversary."
I'm reminded of something that Miles and Wood wrote in the fifth volume of About Time
in regards to "The Five Doctors." They called the episode the equivalent of a comic book crossover or a Star Trek
convention. The story was, basically, a thin excuse for bringing all the old faces back. (Now, I should say that I enjoy "The Five Doctors" a great deal, and the story generally works for me.)
I suspect that Moffat is treating the special in the same way he approaches the Christmas specials; he has a vague tie to the date, but then goes on and does something else. (I'm not sure what "Wardrobe" and "Snowmen" had to do with Christmas, to be honest. But then, none of RTD's Christmas specials had anything to do with Christmas, either.) So I think Moffat will trot out a bunch of things from the past -- scarves, Daleks, Cybermen, etc. -- for quick cameos, and then he'll get on with just another story about the eleventh Doctor and Clara.