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.
Yeah, I think Moffat also treated John Barrowman with crap too. I heard rumours that he was asked to come back for series 6 (probably to join Amy Rory and River in episodes 1 & 2) but turned it down because he had scheduling conflicts, so couldn't' do it, and after that Moffat's in a sulk and that's probably why he's not in the 50th. So from what I've heard of how Moffat treated Eccleston and Barrowman, no wonder Eccleston declined. From what I've heard, Moffat sounds like an impatient git who writes only for himself.
Or maybe Eccleston like myself and many others are fed up with all these giant plot twists and complicated shit that goes on. And with Moffat, he always marks an occasion with some stupid "altering the universe with complicated plot twists" crap that Eccleston may not like, and I get the feeling he's a bit more down to Earth with the show.