Well, it seems that Blizzard will keep adding gear with the property

Decreases magical resistances of spell targets by <amount>

where <amount> is usually 10 or 20. I'm assuming here that this, just like Warlock CoE/CoS, is not able to reduce resistances below zero.

A long, long time ago, Scorponok calculated in a post on the official forums that in the long term, 10 more resistance to a spell type means 2.5% less damage taken. So, assuming that your targets have more resistance to your spells than the amount you reduce it via gear, for every 10 resistance reduced you gain approximately 2.5% DPS.

This is great of course, but how much resistance to different spell types does Ye Average Trash Mob in a raid instance have? Does anyone have any ideas about that?

[Edit: grammar.]

It is apparently bugged in PvE.
http://forums.subcreation.net/viewtopic.php?t=482

Binary spells like Frostbolt work fine though.

wrong, i modified the glancing hits addon that was posted a few weeks ago on the rogue / warrior forums to monitor partial resists from spells in a boss fight like huhuran for example i lose about 5-7k damage to partial resists alone.

That's funny, I just had a conversation with some guildies about spell penetration gear, and the analogy I used was that partial resists were the glancing blows of spells, and -resist gear is like +skill gear :)

This is coming up on a lot of threads around here. I think everyone's looking at Mages more closely after the review, and perhaps my Mage theorycraft posts called some attention to it as well.

Basically, we don't know exactly what's going on in terms of resists. It would be nice if we could start getting some really good parses.

Like Maledict says, this is happening due to level difference, read more about it here: http://forums-en.wow-europe.com/thre...n&T=132218&P=1

No matter what -resist gear you use or what crazy debuffs you throw at a mob, who is at least one level higher than you, you will get occasional partial resists.

I have two theories about this:
First one is that it might be a unintended leftover from the leveling days to prevent people from killing mobs that are alot higher level than themselves. Holy spells and such are recieveing partial resists due to this, which proves that it is indeed something out of the ordinary.

Second theory I have, is similar to what Twid said, that this is our intended equivalent to glancing blows. The only thing that contradict this is the fact that hunters don't get anything like this on their ranged attacks.

Whatever the reason is, It would be sweet if they made spellpenetration affect this in a similar way as +skill, seeing as spellpenetration is currently a completly wasted and useless PvE stat that only eats off the item budget.

Mobs do have holy resist, its players that can't get any - it was mentioned in a blue post on the WoW forums, I can find it if you want. Or, you could have a hunter info (don't remember the name of the ability atm) a mob, there's a little slot for holy resist(unlike for players).

I'm also fairly certain level affects only the binary "to hit" resist for all spells (white resist? I never remember which is white and which is yellow), which can only be countered with +spell hit (or the talents which read -resist % to spells) - which would also mean any time you see a partial resist, there are resistances working against you past CoE/CoS (neither of which helps for holy damage, if I'm not mistaken) if they're on the target, and -resist gear would be useful.

Just read the post I linked to above. The dude that tested it used a total of -245 spellpenetration on a boss and still got partial resists (around 5% damage lost). Do you honestly think some bosses walk around with 245+ arcane resistance? Do you honestly think that he would only have lost 5% damage if the boss indeed had 245+ arcane resistance? I should also mention, he used arcane focus + his normal hit gear, which brought him close to the 16% cap. Or do you think you need -315 and more than +14 HIT to negate this?

This is level based. It's been proved numerous times, and NO STAT AT ALL can currently affect this.

Go shoot any level 61+ mob or boss with a non-binary spell. You will not get partials every time, but occasionally small partials. Then debuff the target with CoS, equip mass spellpentration and hit gear. You will get the same results.

Sadly, I don't really have access to gear/classes that can create situations to test this further :(

I would like to know more about this, however. Does this weird partial resist have a binary counterpart? Is it relative to level difference (does it happen more often on 63+ raid bosses than on level 61 elites in silithus?) Do talents that are -resist% to spells (such as the new mage talent, or the shadow priest talent) affect this, or does it function identically to +hit% gear?

Knowledge of the rate of this 'weird' binary resists would prove useful in gathering data as to the real use of -resist gear in pve: if they are partially resisting past the "known rate" of 'weird' resist, then obviously -resist gear would come into play.

Edit: Missed a part of an earlier statement which answers what I meant to ask with the last question, despite phrasing it badly.

My personal experience is that it occurs just as often on a level 61 target as a 63 target.

The priest, warlock and mage talents are equal to the +hit stat, which works off level difference but has nothing to do with partial resists.

Level 63 mobs have a chance to partially resist spells on top of resistance/level checks that amounts to 5% total damage mitigation, which cannot be removed in any way, even if you have 17% hit and the mob has 0 resistance.