Make 1-20 slighty harder. Nerf exp chains and links a bit. Call it a day.

Currently, someone who knows what he's doing and can only play 1-2 hours during the week and more during the weekends (IE, the play style aimed by Yoshi-P) can reasonably cap one battle class in a bit less than a month with no PLing. We have 7 battle classes, so we can sorta round it down to 3~6 months to level all battle classes (remember, no PLing, no 4h+ with a static party a day every day). Then we have crafting and gathering. That alone is enough for literally a YEAR of doing NOTHING OTHER THAN GRIND. Let's not even forget that 50 is a temporary cap.

This, however, assumes the player knows what he's doing. For a new player, the first class will likely take 2+ months unless he is PL'd. During that period, he will also have to find a good linkshell, acquire money, materia, learn to play, do some end game, etc, etc. For the average gamer, the current grind is actually fine. For the gamer with lots of time, hardly any level curve will be enough. These should NOT be the chosen stardard to balance regular leveling around, sorry.

As for "learning how to play in parties", mindless grinding hardly teaches us how to avoid Batraal's laser, rush Dzemael or evade Eruptions. At best, we learn that no matter how far we are from the hedgemole, it's AoE will always hit us. =/