Step 1: Make a very lengthy Tier that has many easy bosses and many hard bosses as well. It will be long and it will stay for long because it is the beginning of the expansion.
Examples: Tier 11, Nax2 + Ulduar. Plenty of bosses. Plenty of easy of bosses. And Plenty of more challenging bosses.
Step 2: Make an intermission raid that is the most challenging of them all without as many bosses. People will have learned their classes and they will also not have time to do as much progresssion since they will have both the previous and the next Tier to deal with.
Examples. Firelands, ToC. Fewer bosses. Fewer totally easy bosses. And one very challenging boss.
Step 3: Make the final raid which will be the easiest. People will feel they drop many bosses - and the nerfs will help with that; nerfs that have started from the end of Tier of Step 2 - but they will be easy to down and one boss will be challenging. Make the people feel they do something but that they complete it as well. We don't want them to feel they go to the next expansion with things left to do.
PS. I realize that Nax is a weird example being put with Ulduar together but I believe it deserves it since it didn't feel as a real release, it was mostly a "small filler" to design Ulduar, so I believe it served as a part of Step 1.
So yeah, I'm really surprised people are surprised DS gets nerfs or is not as challenging or interesting as others. It is by design like that so people don't feel they go to the next expansion with things left to do. They want to make the world feel "done" for as many people as possible.