Ghostcrawler posted in a followup:
You get a small mastery bonus for wearing the highest-armor gear capable for your class. Hunters in mail get a bonus. Hunters in leather do not.
So now we know how they're going to penalize downgearing.
Wow. I won't lie, this annoys me since it's been years and they still don't seem to get how to properly itemize for classes they make and manipulate (cough, moonkin). A lot will have to change in terms of that getting straight for me to feel confident that this won't anger me at a later date. And I am not sure the spirit to hit thing will be enough.
Yeah. It makes me a bit nervous too. With that said, assuming they have 1 spirit->1 hit, it seems likely that we'll be able to wear spirit gear like we wear hit gear now. Now if they make it so that it isn't 1:1 spirit->hit, we'll be having the problem that we'll effectively be "losing" item budget for wearing spirit, which will make it about as useful to have on our gear as it is now -- ie, something you avoid as much as is possible because its effectively lost itemization. And taking a small hit to mastery to get full use of the itemization budget on a piece of gear will be a much smaller loss than wearing spirit gear. At the moment, there is the problem that blizzard still hasn't shown itself willing to generate gear for moonkin.
But really, its way to early to tell anything.
In ICC10, there are 7 leather drops. 3 are itemized for resto druids, 1 for boomkin, 4 for neither*.
In ICC25, there are 8 leather drops. 4 are itemized for resto druids, 0 for boomkin, 4 for neither*.
* Boomkin itemization is spellpower/haste/crit. Resto druid itemization is spell power/haste/spirit. Neither really want spellpower/crit/spirit - though very high geared resto druids might be able to make use of crit as a lesser stat. Crit is a decent resto stat with 4T9 which I suspect confused blizzard when they saw resto druids wearing crit.
It occurs to me that if blizzard had just made "boomkin get spirit->crit at 1:1" as a talent, they'd have only needed one set of gear for both boom & tree.