Well, Emerald Nightmare had interesting fights, not sure how Highmaul was, but yea they usually release an easy raid for the first tier and then ramp it up from there. That goes all the way back to Naxx in WoTLK.
Highmaul was fucking garbage. I really didn't mind the questing in WOD and the story lines that went with it, actually rather enjoyed it and for my first time leveled up two toons, but once you hit that first raid you're like wtf... The first, what, three fights are in the initial arena, and then three bosses you can fight in any order that are all very uninspired, all of which are optional, and the final boss fight was basically a target switch encounter. Bladefist was really the only one that had interesting story or history, and he was a dps with adds fight, and the first boss so the easiest... Blackrock and Hellfire were admittedly pretty good raids, but only three raids for an entire expansion was exceptionally light.
Legion bosses were leaps and bounds more interesting in both technicality and difficulty, in all of the raids. Cenarius, Xavius, Helya, Gul'dan, Kil'jaeden, and even Argus were all very involved, challenging fights, with a ton of history, lore, and associated quests. That's what Legion did exceptionally well. I thought the Lich King was basically the end of Warcraft, but Legion actually was. BFA is gonna be that unnecessary sequel, or 11th season.