It does absolutely not make any sort of sense that the first 30 levels are slower than the 60 last levels. While I personally am of the opinion that this is because abyssea leeching is dumb, I also understand that nerfing abyssea leech exp is now not a real option. It has been going on for too long for it to be sensible to stop it, and it would only put new players at a disadvantage. They would be the only ones without 15 level 90 jobs, and have to spend forever getting their jobs up.
The only sensible solution to this obvious inconsistency in leveling speed, is to make it even faster to level outside of abyssea. I think that the exp boost that was done recently helped with getting the per-kill exp gain pretty all right. However, we all know that it's not just enough to get a huge lump of exp per monster, but each monster also needs to be defeatable pretty fast.
A fair solution would be to simply make characters stronger relative to the monsters in vanadiel at levels between 1 and 75. A vanadiel-wide constant and potent regen/refresh could be one thing to add. Removal of the exp penalty for having adventuring fellows could be another, along with making them smarter, and being able to equip them with more gear that would increase your duoing effeciency. This would let any job make up for their weakness in battle by configuring their NPC correctly. Considering more and more exping below level 70 is done in just with a few friends, maybe make the exp bonus for small parties even bigger.