Oddly, the one thing we do know for sure about Eccleston’s leaving is that it wasn’t due to fears of being typecast. When the fact that he was leaving was leaked to the press, the BBC rushed out a press release which gave that as his reason, which he forced them to retract.
Neither Eccleston nor any of the production team have ever said publicly what the real reasons were. Reading between the lines, it seems to me that he may have been having a bit of a stressful time personally, and the shooting for Doctor Who getting off on the wrong foot may have made it more than he felt he needed to deal with. That’s just my speculation, though.
Christopher Ecclestone only agreed to do Doctor Who if it could only be for one year. It was known he was leaving at the end of the first run before they began. He didn’t want it to take over his foreseeable life. I have an official book that explains it, somewhere around here.
I’m a big fan of Christopher Eccleston, and he rarely stays in ANY part for more than one year, even when asked repeatedly to do so. My own wild guess is that he doesn’t like repeating himself.
Army of Ghosts/Doomsday from Season 2 is essential.
**Bad Wolf/Parting of the Ways **(Season 1 finale)
Of course, both of those two-parters, being finale episodes, tie in greatly to their respective seasons, so to really get all the small details you’d have to have seen all that came before. But those specific episodes are greatly referenced during Season 4. The Runaway Bride (first of Season 3) as it introduces Donna.