Major characters definately died in Angel, but not so much in Buffy. In Angel the core group was always changing, and it benefited from it somewhat. I mean, by the end, Angel was the only person who was there at the start, but I wouldn't trade Wes and Illyria for anything. On Buffy, however, they built up the group from the beginning. Fringe members came and went, and Tara (although a main character) never quite made it into the core group. First episode we had Buffy, Giles, Willow and Xander, last episode we had Buffy, Giles, Willow and Xander. If one of them (except possibly Giles) had died and didn't come back, the balance of the show would have been severely compromised. Whether this would have been for the best I doubt.
It worked in Angel because of the slightly darker tone combined with the fact that the threat was always there after Doyle. There was always precedent. Joss tried to establish that in WttH with Jesse, but it never really worked. Buffy was a much more optimistic show. "What cant we face if we're together?" and all that. (Although that's not a great example considering what Buffy really meant...but you get the point.) There was always the feeling that the good guys are always stalwart and true, the bad guys are easily distinguished by their pointy horns or black hats, and, uh, they always defeat them and save the day. No one ever dies, and everybody lives happily ever after. Not always the case of course, but it's that general mood that makes eps like The Body really stand out. Angel was never as clear cut, so main character deaths were much more acceptable to the viewer.
