Trading between alts

[SamSmoot]

Because you don’t need alts, ever? Every class can be done on one character.

Storage space? If you try having a few crafts and gathering professions, you'll be perpetually out of storage space.
Personally, I'd have gathering profs and DPS on one character (So I can kill troublesome mobs faster), and my crafters on one or more alts.
When I tried fishing early on, I nearly filled my bag space with just baits and different fish.

Edit: I'd like to see each gathering and crafting job to come with a new bag that's automatically included in your standard bag list when you switch to that job, but is restricted only to items used in / produced by that job. Crafting items would be used from the crafting bag, or the regular one, with no need to move them first.
You could still access it from other jobs like the chocobo saddlebag, to remove or add compatible items.