And I was able to verify that the AoE penalty is now lowered to 50%. However, I noticed that the AoE cap was implemented in 2.0.3. Specifically, my arcane explosions were hitting for an average of ~650 while bombing the trashpacks next to Patchwerk according to SWS (but that was with AP and Hazzarah's Charm of Magic), while they were hitting for an average of ~180 while bombing after the decimate hit fighting Gluth (only with AP). Quite a noticeable difference.
First, the base AOE penalty is now 1/2 instead of 1/3.
Second, if an AOE spell lasts for 8 secs in some way, it gets multiplied by 0.83 in addition to the 1/2 AOE penalty. No idea why, but this fits consecration, rain of fire, flamestrike's DoT part and blizzard. This might just be a bug, or it might represent another way to calculate the coefficients for which a cleaner formulation isn't clear yet.
Now the question is, is there a cap based on a fixed damage number per spell (e.g. 5000 for arcane explosion, 8000 for blastwave, ...) or based on a fixed number of targets it's supposed to hit (e.g. full damage against 10 targets, 50% of full damage against 20 targets)?
Could anybody test this already or found some reliable information? Or does anybody know how many zombies you're bombing in the Gluth encounter, so we can get a rough estimate?
One other bug I noticed is that arcane missiles seem to very often lose one missile. I then tested this in duells and it is independent of the rank of arcane missiles you are using. I got a rough 50% of casts that were missing one missile, giving me an effective 4.5 missiles per cast of arcane missiles rank 3-8. Nothing was hitting me, and I got 5/5 improved arcane missiles anyway. This basically reduces their dps and dpm by a staggering 10%. Quite sad.
Edit: Also while doing some test duels against a protection paladin, I noticed one of my fireballs hitting him for a measly 851 fire damage, none partially resisted (I double-checked the combat log). This fireball was casted with arcane power. All the other fireballs I hit him with hit for the normal ~1500 damage, the arcane power fireballs for ~2000 as they should. Why this one particular fireball hit for only 851, I have no f*cking clue.