Guild Wars 2 has a system in place that allows higher-level characters to revisit low-level zones. The player's level is scaled down to match the zone, so they can still get the same challenge. This means they don't have to miss out on content, and they can jump with friends at any level and not be the overpowered giant in the group. It keeps it fun for everyone involved.
Changes are coming to this system to try and make it better, such as making high-level characters less powerful. Sure, it's fun to kill everything in one hit, but it can get boring after a while. Characters will have a better challenge and it should fit in more with the intended difficulty of the area. Rewards are also being tweaked to make it worthwhile for players to return to these zones.
Dynamic Leveling Adjustment [Guild Wars 2]