You realize, of course, that since Eccleston DID take the meeting with Moffat, that he was most certainly told what the episode was about and maybe even shown a few pages of script.
He's an authentic, true guy.
Maybe he turned it down because it looked terrible, or because he felt all this "The Doctor is really an evil monster!" crap was...well, crap.
For people who ARE NOT excited about the 50th based upon hints at plot, Eccleston turning down the 50th is very likely good validation that it is a terrible, hurtful story. More Moffat crap.
Also, Moffat seemed very impatient and irritated with Eccleston, and I totally believe that Eccleston showed up, and Moffat treated him like crap, and Eccleston said screw it.