I like the idea of actually having a choice in terms of pieces of gear, but it seems at the outset that this is going to get very complicated, both for you designing them, and us picking things (and for me doing the list.

). The reason for balancing it coming up is simply because it is not too small a benefit to be noticed on some classes (going off on procs for melee's for example) whereas on others (casters, who do a large amount of damage over an extremely short amount of time) it adds a veritable shitload.
Example:
Monk Bard or w/e does 100 points of damage on their proc, whatever it is
Wizard does 5000 points of damage with their moon comet
Mage does 5400
Necro does (idk whatever the fuck dot they're using so lets just say) 4400
Over the time these are going off, as mage and necro are delayed, we see this (with 100 elemental on each of the particular type they are using)
Monk Bard or w/e gets a x2 increase
Wizard gets a .02% increase
Mage gets a .018% increase
Necro gets a .022% increase
Is it overpowered on the casters? No, it's underpowered. Is it absolutely massive for the proc? Yeppers. Thus adding to a problem of balancing it, I mean if I'm getting this right, the change was intended primarily to benefit casters, and ends up benefiting us less than other classes. That is at least what I got from the balance issue.
Part of the problem here isn't with the effect itself. It's with communication. Players have a very nebulous idea of what this will entail, as we obviously should at this point, since it's not out yet. Though the heads up is always appreciated, as it gives us time to actually discuss it before it goes in. Something I feel is useful. The only real examples of elemental damage we have to look at are ones that we see through previous items. Most of them are fairly tiny, and when there is a large amount on an item, generally that item is very subpar in other areas such as hp or mana. In light of that, with the very small % increase that would be seen on casters whom it is geared towards, I just don't see it being worth much of a sacrifice if it follows how I at least am conceptualizing it, especially in light of very expensive spells. 100 damage sacrifice for how much basically is what it comes down to. Since we don't know the specifics, and should not until they're out, all we can really do is speculate, which can be valuable, and really overall is where forums shine.
One of the caveats with this is why not just add energy defense spec to items instead? It would be comparable if we're capping it. It's an interesting idea, I feel it needs just a bit more to be comparable even to that (and energy/defense + specs value was very debatable). So how about this.
In line with the multiplication idea, Damage increment (and yes affliction enhancement too, I didn't forget about necros) is already powerful enough. Without it, you are a piss poor caster (unless you are an enchanter, in which case it mostly has no value). However, elemental foci are not nearly as powerful...and this is elemental based...so would it then be possible to multiply the damage being done by this added elemental damage by an amount going along with the foci of that type? This way you would see something like (arbitrary numbers)
4 ice on a bracer
Shojar's cruelty on
4*6=> 24 total damage added.
This is substantially better than 4. It also will be more likely to benefit casters (the ones I'm assuming this change was mostly geared towards) as they are the ones most likely to have some elemental foci.
The reason for the suggestion on this in part stems from what I see on various gear setups. There is very little impetus for many classes to acquire all the elemental foci of the best nature they can get, look at the top 5 necromancers for a good illustration of this. 2 with poison focus whatsoever, 3 with disease, and only 1 of those with 6 disease. Another good example is 1 of the wizards having magic focus (albeit magic focus pieces tend to be a bit rarer). This might actually up the value of elemental foci to be at least close to in line on all the types of damage one can do, increase diversity in terms of gear selection, as well as giving this whole idea a better benefit. It would also fit with a cap of 100, as 400 would then be the highest one could get as a bonus (unless we're seeing 8% elemental foci too come ikisith). As it currently stands, an item from early tmaps provides me with about as much bonus as I'd see from stacking up 100 elemental damage, with an ostensibly lesser sacrifice (a buff slot rather than a gear slot) so I don't feel a maximum total damage bonus (after multiplication) of 400 would be too high (again I am making some assumptions about the amount present on gear, it might be a little hairy in terms of necros to actually balance that, as one could add a hilarious amount of dps with particular selections on them, though not really with wizards and mages). Putting it another way as well, the ones who most likely are going to be using a particular type of damage over and over again, are wizards. Mages and necros use varying types to acheive their dps, so in terms of overall benefit, this will actually end up even lower for us and our deadlovin brethren.
And yes, we already have to balance as casters a truly ridiculous amount of things. It's at least a good 11 or 12 foci for necros and mages, then the 20ft. More often than not alot of gear is passed up simply based on a rarer foci on that already particularly strong slot, beats gear for the next few tiers.
Eh the problem with caster 2hers is still going to pretty much be the same as it always was, I'd love to see a worthwhile high tiered one, currently I don't feel they exist. Benefit for benefit. Casters are already extremely lacking in hp, and the large amount of damage we end up taking in the endgame even when we are not attracting the main mob's intention is generally fairly considerable. Thus it is fairly understandable why many would not sacrifice 100 hp even for 100 extra points of damage. I certainly wouldn't, because I can't do much damage when I'm dead. More fluidity in terms of gear would be pretty nice. Focus effects being so rigid does blow, and oftentimes causes overlooking pieces. Really people are just looking at this like ferocity for casters so to speak, so it's going to be agonized over any way ya slice and dice it (which actually is another interesting idea).
Actually as a sidenote one thing I just thought of....foci like say, experimental runes. Is it possible to say take an effect like that, and actually make it stack somehow with foci that do similar things? Kind of like valiance and ferocity really is what I'm thinking. That would make things a bit more fluid if handled right. Just a thought that popped up in my head when I was reading zae's last few sentences(and yes, I realize it is a pretty nebulous thought, I gotta think about that one some more).
Another thing I noticed here..is there actually going to be a sacrifice on the various pieces of gear for it other than weapon and chest? Because I'm kinda getting mixed messages here a little bit, which probably accounts for most of the problems people see with it. If it's a bit of free damage, sure by all means stack that on like a fat man stacks pancakes. But if we're sacrificing 20m here, 10hp here, some resists etc, then it wouldn't really be free. Which would mean alot of those pieces would end up unused if they're perceived as not worth it (a problem seen with thaz water augs previously, the reason there were so many of them was not because of energy/defense spec I'd wager, but rather because of the previous ft on them, I personally selected advanced effects, hp, and really anything else I could get instead of a few points of damage, and with good reasons for doing so), and unused for previous tiered items. Balancing gear is not an easy job by any means, it's enough of a massive pain in the balls just to judge between characters with the gear. I don't particularly envy your job.