If you haven't played since TBC, all of the 1-60 leveling content was completely redone. Plus there have been four expansions since TBC, WOTLK (70-80), Cata (80-85), MoP (95-90), and WoD (90-100).
In other words, you may want to skip using the level 90 boost and just try starting from 1. Revisiting old leveling zones you remember and seeing how they have changed or what old references & Easter eggs were put in, then experiencing the leveling content from expansions you missed, will probably be more fun than using the 90 boost and rushing to WoD content. WoD isn't great. Starting from 1 doesn't take very long (1-100 takes *far* less time than the old 1-60) and you'll be able to move onto to new content before you get bogged down or bored. You can always save the 90 boost for another class if you decide you don't love what you picked by the time you reach 90.