Re: The Doctor Who primer
It's not mentioned on screen, but it's assumed by most that the Time War is the result of the regeneration. The reason why it wasn't shown is because Davies didn't want the audience to get use to McGann and then five, ten minutes suddenly switch to Eccleston for reasons a new audience wouldn't understand right away (and find it off putting).
"Eccleston was a tiger and Tennant was, well, Tigger. Smith [is] an uncoordinated housecat who pretends that he meant to do that after falling off a piece of furniture." - Lynne M. Thomas
"I'm in Hell and it's full of Avons!" - Vila