I won't argue that any of these are well written, or even consider the rest of the world. Blizz writers have a nasty tendency to focus on the story in front of them without considering the setting. My understanding of their intent is that events of past expansions aren't happening any more even though the quests are still available, i.e. the Duskwood worgen were cleaned up by adventurers years before the Worgen starting zone. What I was getting at is that the Forsaken took place sometime between WC3 and WoW, when everyone only knew the mindless Scourge. Since then, they've learned that's not always the case, as backed by Prissypants apologizing to Sylvanas over it.
In the case of the DKs, two factors apply. First, after the "duuuude awesome" moment, it felt abrupt and rushed. Suddenly capital city! I suspect some bits were cut here. The second factor is scale. We travel in a blink in game, but for reference, Karazhan to SW is supposed to be a week of gryphon flight. Light's Hope to SW should proportionately be even longer, and news is supposed to have made it there before the DK arrives. He/she doesn't get a warm welcome by any stretch of the imagination, but isn't kill on sight. What's the difference? Years of knowledge, news to expect him/her, and Tirion's personal vouching.