Yeah I thought it as underrated too. I was not a fan of Garrisons, but I loved the following:-
Levelling and alts - first time I reached max level on one of each class. I LOVED the way levelling worked in WOD. The pace, the zones, to me it is the best expansion ever released for this.
Ashran - Again I loved Ashran. Amazing for alt fun gearing.
Theme - I loved the brutal atmosphere of WOD. Iron, bombs, Iron horde! Loved the cinematics too.
Raids - Enjoyed most of the raids in WOD. Again loved the metal theme.
PVP - This was the last expansion where I found pvp still bearable as a caster. Legion and BFA have put the final nail in the coffin for pvp for me.
Casual friendly - While hardcore raiders complained, I loved that there was an expansion where I could keep up at my casual pace. I had lots to do as a casual with alts. I have never liked that mmorpgs cater to people who play 5-8 hours a day. It should be illegal imo as it encourages addiction.
Overall I would give it a 8/10.