I had a chance to do the new Caverns of Time instance, The Culling of Stratholme, yesterday. My impressions are fairly negative. It is a rather disappointing instance, at least compared to Durnholde and Black Morass.
First off, Blizz makes it very easy for you by having all the citizens turn into Scourge very quickly. Arthas kills a couple civilians, but the vast majority of the time is spent killing undead. It sort of negates the whole impact of this event. In Blizzard's lore, the slaughter of civilians in Stratholme is where Arthas crosses the line, where Uther and Jaina abandon him. However, this instance pretty much puts Arthas on the right side, as everyone in the city seems to be converted to Scourge already. Of course, it would be hard to have players killing civilians, but that's the problem with choosing this event in the first place.
Second, Blizzard is too attached to the whole tourist angle for the Caverns of Time. Warcraft lore is decent B-level fantasy, in my opinion, but it is not strong enough to support tourists. It really only works when the player is a active participant. Maybe Lord of the Rings Online could get away with this, but I don't think Warcraft can. WoW needs the active threat, the notion that the players are needed, to work.
In the first two Caverns of Time instances, the Infinite Dragonflight supplied the motivation for the players to get involved. In Stratholme, they're pretty much an afterthought. They are only in one small section, as the second-to-last boss. It feels like Blizzard just added them in because everyone complained about Hyjal. You could strip them out and--aside from missing one boss--the instance would pretty much be the same.
So that's my view on The Culling of Stratholme. It's a mediocre instance that's little more than a glorified cutscene of an event from Warcraft III. Rather disappointing, really.