With the current state of content after the retune I think we had all kind of forgotten how hard some of the old fights in the game really were; Vashj is an example of a fight that's still very hard. Looking back in retrospect most of SSC was actually fairly trivial after the retune. We learned Leotheras, Lurker, and Karathress with little more difficulty than most of the BWL bosses back in the day. Hydross and Morogrim were a little tougher, the Vael and Chromaggus of the zone respectively if you will. Vashj however brings back a difficulty level reminiscent of fights like C'Thun and Gothik. It took us a good 8-10 hours of learning to get her down finally, which is more than we've spent on any other boss in TBC, possibly excepting the old pre-nerf Magtheridon.
And furthermore the fight is amazingly well crafted. It's a fun fight, and it's really hard without being terribly frustrating or stressful, which has traditionally been pretty hard to achieve for Blizzard. There are very very few luck-oriented things going on: it's metered, and whatever random elements may occur are largely overcome with proper strategy. Extremely effective communication is also a requirement -- one that seems to be lacking from a lot of earlier TBC content.
Anyway, with Vashj down we now have 25 people halfway down with Hyjal attunement. We'll be pushing to finish off Tempest Keep soon and hopefully get the other half done in the coming weeks.