Since I'm someone who kinda sorta prefers a slower leveling speed I'm going to mention my reasoning.
Most games tend to make end game content the end all be all, and also the most common level for content, rightly so, but when you hit level 20, and there's a level 20 dungeon, with really good gear for a level 20, that becomes outdated once you hit 25, there's little reason to stop and do that content when I'll be able to hit lvl 25 in about 2 hours, and just buy better gear than I would have gotten from the dungeon. As someone who isn't that driven by story (not me btw), why should I take an hour of my time doing a dungeon to try to get a piece or pieces of gear that may or may not drop and I may not even be the one in the party to get the drop.
Level syncing is great, but doesn't really give much incentive to doing lower level content, other than helping out a friend.
The slower leveling makes not just lower lvl content more important, but it also makes lower lvl gear and items more important.
My suggestion would be to give a bonus to those who level multiple classes/jobs, while not entirely obstructing someone who is power leveling a single class to max. For example, how about for every class that you have at every 10 level intervals, your character receives a 5% exp bonus for every other class except the current class?
As a Lancer, if I had CNJ, MRD both at level 10, I would receive a 10% bonus to experience, if MRD and CNJ at level 20, I would receive a 20% bonus to exp for every other class. If I had MRD at 50, CNJ at 30-39, and THM at 20-29, I would receive 50% bonus to every other class.
Slowing down leveling makes everything not at cap more important, whether it's content, zones, enemies, gear. With the way the trends going now, we may as well just remove levels altogether.