Doyle was a major character, the one who came to Angel with visions of people in trouble. Doyle set him on the path of the show and was the vehicle that gave him purpose. And halfway through the first season, dies in a blaze of glory. Literally.
I can't think of another Whedon show where a major character was killed off so soon. I think I'm still expecting Doyle to come back. I mean, Angel got resurrected, Darla apparently is coming back, and Buffy got killed off twice. Even Warren and Jonathan's come back, kind of.
And if Doyle doesn't come back, then that's a pretty big change in Whedon's MO. Thought it was worth a mention.