You would only miss out on the story quests of each class, the mount from leveling them all to 80, the experience of playing them which is fun, the fact different fights can present a different amount of challenge to different roles (for example, some fights are hard for a tank and some of them are hard for a dps). You would also miss out on crafter and gatherer content in the firmament, ocean fishing and certain beast tribes.

None of these things matter a lot unless you are someone that likes to experience everything in the game or collect all mounts and most people do main just one class even if they switch to a different one occasionally for fun.