I wrote a letter with a whole list of things that may or may not be THAT clue.

Will re-read after the first viewing (and probably laugh my head off)!

I like the ring thing. I always thought they were just wedding rings, because in my country, they do wear them on their right hand. The doctor that is feeling Sherlock's pulse also wears one. And besides, the way they throw him onto the stretcher suggests that they are pretty certain he isn't hurt. Or Moftiss didn't do their research on medical procedures. It's just so easy to see all kinds of meaning into things that are just mistakes in the production.

I actually believe that the 'clue everyone missed' is a red herring. They haven't read all the theories so how can they know what people missed? IOU is a red herring too, I think, just a nice motif like the angel en Grimm references. 

But there are things in the episode that are never really resolved. Why did the young girls scream? Nobody ever answered that question.
And Moriarties suicide is totally boss but it doesn't really make sense. Sherlock wasn't saying anything he hadn't said before so why would that tip him over the edge? (It does have another angel reference with the backlight, which is cool.)

 There's a continuity error with the fall and the landing too, (he lands at a 90 degree angle compared to how he falls) could be just that, but since we know there must have been some kind of interruption, it could also have been the clue. 

Anyway, this is a real long way of saying that I have absolutely no idea. Looking forward to ep 1. 

I'm pretty sure the clue was right in front of us during TRF.  John went to Mycroft's office at dawn on the day of the suicide hoax and criticised him for giving info about Sherlock to Moriarty.  But I've been certain for months that Mycroft was the organizer of the suicide hoax, and the thing which Sherlock did that was "out of character" was asking Mycroft for help in executing the planned hoax.

I think Sherlock first realized he needed to fake his suicide just 12 hours before he actually jumped -- when he was in the street outside Kitty's house late at night, walking back and forth and talking rapidly.  He said, " . . . there's only one thing he needs to complete his plan, and that's to -- "

He stops and then says to John, "There's something I have to do."

I think Sherlock came up with the initial plan right there -- not days or weeks in advance.  And immediately after that, he spoke with Molly and then called Mycroft to start the process of organizing the suicide hoax. 

So, the clue which everybody missed was the fact that Holmes was willing to ask for help in executing  his planned suicide. His extreme change in attitude made the plan possible, with the help of Mycroft, Molly, and all the people they employed to conduct the operation.

I know I'm not the only one to present this idea, but there are many people who seem to be looking for tiny details they can turn into dramatic revelations, and I don't think these theories are the answer to the quesiton.