Expectation: we’ll get to fight Mephisto. VoH: we fight an avatar of Mephisto. For all its faults, the base game had Lilith feel like an actual final boss to the story and Uber Lilith is still the most challenging fight in the entire game. The Mephisto wolf doesn’t even have an uber version of the fight. I think that people were hyped to fight the apocalyptic, colossal form of Mephisto that was shown in the cinematic trailers leading up to VoH’s release. The wolf would have been a great mid-campaign boss fight but definitely not the final boss of the entire expansion.
Expectation: VoH will continue the story of fan favorite character such as Lorath. VoH: Lorath narrates the campaign but is nowhere to be found. I don’t know WHY you would sideline arguably the main character of D4 but here we are. Donan would have been great too if you hadn’t decided to write him out of the story in the base game.
Expectation: Urivar will be the secondary villain of the story. VoH: Urivar disappears for 2/3 of the story. The schism within the Cathedral of Light felt like an afterthought, not one of the main plot points of the expansion.
Expectation: Nahantu will capture some of D2’s dark and moody Act 3 atmosphere. VoH: Bright and sunny.
Some of Blizzard’s best stories were very straightforward but are now considered to be timeless classics (SC1, D2, WC3, WotLK, D3, MoP, Legion, maybe even the base D4 campaign). Meanwhile, VoH has more in common with BfA and SL.