I agree, the introductory heroics were great, and I love the Zandalari heroic dungeon remakes. I felt that the expansion started out strong, with Blizzard appearing to take a heavy stance that they wanted difficult content. Then, they lost a large number of subs (I think it was like 300,000 or something), and many complaints on the forums caused them to cave and give slightly easier content with Firelands (excluding heroic because I heard that was still pretty difficult). Then they went even easier, with 5man zergfest dungeons in 4.3 as well as a zergfest raid. Pre-nerf maybe DS wasn't as easy, but still easier than the other 2 tiers of raiding in the expansion were.
Basically, they started out with a lot of momentum, and lost it, and proceeded to deliver what felt like mediocre content.