While the other content was added with Siptah update, one cannot lump “Stormglass Set” or the many things added with the Siptah update at first, like caged thralls you rescue and etc as “Siptah content”, because they can be used on the Exiled Lands.
Those cannot be lumped on what makes “Siptah great”.
Well, it is true that it is a Subjective Matter if Siptah alone is a good map or not, but there is something that is not “subjective”: Purpose.
Take the maps for the levels alone, their design, their systems.
Exiled Lands accomplish what it is meant to accomplish. It does not fail into being what it was designed to be. Its places are congruent with its narrative argument, and its structure nudge players in the correct way, allowing someone to simply “go and explore it”. After a while one can go around the map by its logic without using a map even.
Siptah is not a map that accomplishes what it was made to accomplish. It was meant to change some fundamental aspects of the gameplay. They said Siptah was a new way of playing CE, and it might “arguably” be a mere change of scenery and a paid cosmetic improvement of the environment and post effects, but not a “new way of playing CE”. If one says it is “just the same as the Exiled Lands but prettier”, it already fails if that is true.
But Siptah is not “just like the Exiled Lands”. There are many settlements in Siptah. Is there a “multiple pasts” tale with very specific hints and “arcs” you can explore on each one ? Or conveniently, everyone is looking for the same thing the same way ? Do you have different “dungeons” and “adventures”, or the vaults are essentially “copies of the Dregs” made look prettier, with a Cinematic in the End, which even tells the story of the Exiled Lands, not Siptah actually ?
In general almost everything you find in Siptah is a simpler and sloppier version of what there is in the Exiled Lands, but shinier and prettier. Even the curse wall, it is there, but changed to a “mist wall” that kills you of exposure should go away too far.
So, there isnt really something to say Siptah is great, and if one argues there is, it only adds to the legacy of the Exiled Lands, because it seems Siptah is a giant plot device to explain what the Exiled Lands are.