The new transmog system is one of the most eagerly anticipated features of Legion. Working as a part of the UI that doesn't require you to hold on to every piece of gear you might want to 'mog in the future should free up an absurd amount of space, and going account-wide will make it easier to farm up the looks you want.
There will be some restrictions though, with the big one being that any class is going to only be able to "unlock" the appearances of gear that they can and would equip. By "would," I mean that a Paladin, who yes technically can equip cloth, leather, or mail armor, will only be able to unlock plate armor looks, because ultimately, that's what a Paladin should be wearing.
The intention is pretty clear here. If you love playing casters, and so your top characters are a Warlock, a Mage, and maybe a Balance Druid, you're only going to be able to soul bind cloth and leather gear. Meanwhile, if this restriction weren't in place, a Paladin, Warrior, or Death Knight would be able to collect all the gear looks they wanted, supplying them for Shaman, Demon Hunter, or Priest alts.
But there's a hitch.
You may not have played a character under level 40 for a long time, and if you had, there's also a chance that you were playing a Priest, Warlock, Mage, Druid, Rogue, or Monk - for whom that level 40 threshold is more about getting a new mount.
Back in the early days, level 40 was a really big transition - you'd start doing a lot more jumping around from zone to zone and to help you do that, you'd get your first, slow 160% speed mount. But on top of that, Shamans and Hunters upgraded from leather armor to mail while Warriors and Paladins upgraded from mail to plate.
And they still do. There is currently not a single piece of plate gear that you can get before level 40, because no one who is lower than level 40 can equip it.
Well, that's changing.
In Legion, Shamans and Hunters will start out with Mail gear, and Warriors and Paladins will start out in Plate.
Sounds simple enough, but this could have really huge consequences.
First off, this means that there need to be new quest reward options for everything from 1-39. Likewise, randomized gear and dungeon drops will need to be re-itemized. You'll never want Strength Mail to drop, because neither mail class can use that stat.
Now I suspect that they could just flip a switch on existing low-level mail to make it officially plate, but that still leaves you with the need to make a whole bunch of new mail gear, now that Shamans and Hunters will not be sharing with the three non-DH leather classes.
And since this is all about transmog, I also wonder about models. Currently, the mail armor that exists at low levels has more of a foot-solider look, appropriate to Warriors and Paladins. But mail at higher levels has tended to be a little weirder, reflecting the mystical shamans or the improvisational hunters. Simply flipping that low-level gear to plate might work ok, but some of it very clearly looks like it's made of rings. Now sure, Blizzard plays fast and loose with the looks of their armor, but I really wonder if they'll use existing, older assets or if they'll have to come up with new stuff for either the plate-wearers or the mail-wearers to use.
And then there's professions. Leatherworkers start to be able to make mail armor at around level 40 (or rather, at a skill level that you'd get to around then if you level up your profession with your toon,) while still obviously continuing to make leather armor into higher levels. Blacksmiths make a pretty serious shift from Mail to Plate, clearly intended to keep in step with the plate classes for whom the profession makes the most sense.
But this change would, I think, require a bit of a redesign, allowing Blacksmiths to make Plate armor from the start, and maybe even removing mail recipes from it entirely (many of their "mail" pieces already look plate-like enough, so that's not too difficult.) Meanwhile, Leatherworking needs to be able to make mail stuff immediately as well.
So ultimately, this fairly small change could have some serious reverberations through the low-level game. It's still probably worth doing, but I'm curious to see how Blizzard addresses it.