Currently in WoW, healer and caster gear overlap. Both roles use Intellect as the primary stat. Intellect + Hit gear is considered caster-only, Intellect + Spirit gear is considered healer-only, and other Intellect gear can be used by both roles.
What if healers got their spell power from Spirit, rather than Intellect?
That would make Spirit a primary stat, not a secondary stat. It would make the division between healers and casters more obvious, and would have them chase their own unique gear.
From an aesthetic standpoint it would be nice, as each "attribute" is now a primary stat. Instead of having four of them be primary, and one secondary, which seems weird. It would be a lot easier for Blizzard to control healer mana regen, as both spell power and mana regen would key off the same stat.
It would prevent mistakes like when a healer takes a hit piece of gear, or a caster takes Spirit gear. It also wouldn't be that hard to adjust current gear, as a lot of healer gear has Spirit already.
Spirit gear would also balance melee gear. Right now, melee gear has a hard split between Strength and Agility gear, while caster/healer gear has a soft split. A hard split would make the number of people vying for each piece more equitable. Compare the number of people in a raid who want an Agility ring to those who want an Intellect ring. Usually there's twice or more people gunning for that Intellect ring. That means that the Intellect ring has to drop more often to keep things even.
There are several downsides to this idea, however. The biggest is that it does create several new categories of "single-spec" gear: Intellect leather (balance druid); Spirit mail (resto shaman); and Intellect mail (elemental shaman). As well, paladins would no longer be able to use caster swords. Druids and Shaman would need to collect separate DPS and healing sets.
It's probably too big of a change for Pandaria. But it might be a worthwhile change for the next expansion, especially if we get a new spell casting class that wears mail.