There are various upsides to this imbalance. Max level players are able to complete lower level content with ease, allowing them to swiftly complete missed quests and achievements in earlier zones, and can travel through zones without being constantly attacked, since mobs will only attack players several levels higher than them if walked directly into. Max level also makes it possible to solo earlier group content such as dungeons and even raids, making farming loot such as mounts far easier, and for those interested in a challenge opening up a new subtype of play at slightly higher levels.
Most classes in World of Warcraft start at first level, but Death Knights and Demon Hunters are exceptions. They start at level 55 and 98, respectively. This alone can shave a huge chunk of time from your leveling experience. Of course, it only works if you want to play a Death Knight or Demon Hunter, but they’re both solid classics that can dish out damage or tank in a group.