Variance in player skill is hurting FFXIV's itemization design.

[Noxifer]

Personally, I think it should just be done through materia

Oh I agree completely =) Like for example the idea with having it be special materia (Untradeable, bind gear on meld) bought through the limited tomestones. (Or some other system that would limit the speed of access without making it 'exclusive' or extreme RNG.)

But if it was added in the form of "set bonus when wearing x pieces of this set", then both raiders and non-raiders should be able to take part in that.

Personally, if it was added through set bonuses, I'd like to see a little bit more varied bonuses than what exists today for GC equipment.
I mean, as in for example:

If 3 or more Plague Bringer pieces equipped: [minor effect]
If 5 or more Plague Bringer pieces equipped: [minor-to-medium effect]
If 7 or more Plague Bringer pieces equipped: [medium effect]
If 9 or more Plague Bringer pieces equipped: [medium-to-large effect]

(How many pieces needed of course depending on how many that set has, so maybe 2,3,4,5 on a set that doesn't have accessories.)

It'd mean new variables to take into account when mixing sets for BiS. E.g. "Is it worth taking this item that has Parry instead of Crit, if it means I get that +5% bonus to enmity?"

I'm certainly not saying this would be easy to balance; that'd probably be easier with Materia. But it could be very interesting.

Edit: Saw your edit =) Seems we were a bit on the same lines :p But I also think that it would take a lot of space in the tooltip window to list all the effects for all the jobs. If they decided to go that way, they'd probably have to make it into stuff like "Alexander Materia of Healing V" to limit the number of jobs listed ^^;