I could have even forgiven his absence if they'd been able to get him in the studio long enough to finish the War Doctor's regeneration sequence. All I needed was for Eccleston to look up and say "Fantastic!" But no. Instead we get old footage of him re-purposed along with old footage of all of the Doctors. It was fine. It worked. It just wasn't satisfying.
And I realize that Peter Capaldi wasn't cast in time for this to be possible, but what would have made the show truly epic would have been to add 30 minutes, include Nine, AND include Thirteen. (I'm going to assume he will be known as Thirteen, but there will be no retroactive renumbering of Nine through Eleven.) There's no reason you have to introduce a new Doctor with his regeneration. You could have brought him in from the future. Maybe that would have been too distracting, but I think it would have been brilliant. At least they worked him briefly into the climax.
Then there was Tom Baker. I spent the whole episode assuming the asthmatic with the scarf would somehow turn out to be him. Then he pops in at the end. I thought it was a clever way to work in one of the older actors, even if he did ham it up.