Haha
Yeah, Sindragosa was a major step up from the previous bosses, but compared to the Lich King she was a puny little lizard.
M'uru required insane amount of coordination and decimated the raiding scene on my server, and it was very robust prior to that. It never really recovered to the same level, sadly.
Four Horsemen owe that in part to dumb gear requirements, but there is no doubt they were more of a wall than KT. I am led to believe that Spine was much harder than Madness too.
Sindragosa was a wall, but never really of the same magnitude as the others.
4h Was complicated, but KT was a vastly more difficult fight. Heck, Gothik was more difficult. Sindragosa was in NO way harder than Putricide, or even close to Lich King.
And while M'uru is the famous killer boss, once it was fixed/nerfed it was probably equal with KJ at worst. KJ was a very difficult boss, just slightly easier.