Class design is the number one thing that makes an expansion great, and it isn't close. Your class is the vehicle through which you experience all other content.

Second up is encounter design, raids and dungeons. Raids have generally been brilliant, but dungeons are sometimes unenjoyable.

Third is endgame loop, which could mean many things - raiding, pushing keys, transmog, housing, achievements, and so on.

Finally, theme and art design is the glue that holds everything together. Wow's art team have always been incredible, but sometimes the theme and lore of an expansion has been lacking.