An issue comes to mind when leveling is quick and short, which is the issue of equipment.
When the time it takes to level from 1 to 50 is shorter than the effort it takes to HQ+ and meld all the materia for said low and mid range gears, it devalues the crafter's purpose.
I remember in another mmo, where I've invested in an expensive level 1 item with major enhancements for it, because it would shave time off leveling up for multiple classes.
However, in FFXIV:ARR I wouldn't spend a gil on any level 1-40 top line equipment at all, because the time it saves for leveling up is negligible.
The time it takes for the market to collapse for low and mid range gear will be in aspect to how fast leveling becomes. Simply because there isn't going to be a demand for it.
The current minor work around to that is level-cap content along with gimps on scaled down equipments. Although, I don't think the scaled down is high enough to discourage players from using high level gears yet.