I devoted most of Friday and Saturday to playing this expansion with my group. So that everyone else can have a more enjoyable experience, I've compiled things that either were issues for us or at least seem counter-intuitive.
Individual interpretations
Bloodletter upgrade : When I initially saw this card, I felt that it was a true/false statement for each region. Another friend of mine showed me that the wording is extremely similar to the old Chaos card Ritual Slaying. It uses "each time" rather than "for each region." Under that interpretation, I managed to get 22 points in one round, though I still wasn't able to outrace others for points in the end. It also doesn't matter what else is contributing battle dice; just that a Bloodletter exists where you're killing things.
Vengeance: Not so much a rules issue, but a stern reminder. For the first 3 games we played with this upgrade, we forgot about it for at least one round (the first one went without it for three rounds after it was taken). This makes up for the cultists not being able to attack anymore, so if you're having as much trouble as we did, I suggest putting it on the board with a figure where space permits.
Skulltaker: This Chaos card makes dead figures turn into your domination value. Remember, it doesn't matter whether Khorne's the one that kills the figures. In fact, if there's a surplus of attacking figures in the region, you could play it without even being around there and potentially dominate it with their efforts.
Rampage: This very unique card makes an instant battle in the Summoning phase. I was quick to point out that if you have multiple copies of this card, you'll only get one dial advancement in that region from the Summoning phase. Multiple Rampages will not give you more dial.
Lepers upgrade : Keep in mind that this is a true/false statement, not an "each." You'll only get 1 bonus corruption per region with this upgrade, not 1 per cultist.
Plaguebearers upgrade : This upgrade adds corruption during the battle phase, so you could theoretically get 2 tokens from the same region: 1 from the warriors dying, 1 from whatever you're doing in the corruption step. If these guys are your only source of corruption when the region gets ruined, you do not get the top ruiners points. That is checked in the corruption step only.
Great Unclean One upgrade : Like Bloodletters, it doesn't matter how many other warriors you have fighting there, just that the Great Unclean One is there while you're killing things.
Cavalcade of Decay upgrade : It doesn't matter who controls said cultists, so count opposing cultists as well as your own. In practice though, I've found this unlikely to work out.
Infested Colony upgrade : Although domination is handled separately from corruption, they are both in the same phase. If you were already getting 2 corruption during the corruption phase, this will not give you another dial advancement counter.
Quicken decay: There are quite a few effects that summon multiple figures at once, be they from summoning, moving, or placing. In most cases, you should only care about figures entering or leaving and not care about summoning. The only exceptions I could see are Bolt of Change and the Horned Rat's dial advancement bonuses (the card's gone by the time he summons with that).
Filth: You are not adding corruption. You are simply changing what corruption is there. Think of it like changing control of the corruption. Therefore, it does not work for your dial advancement.
Acolytes upgrade : From the way it's worded, this will only work once in a battle phase at all, regardless of different regions. That means only 1 total Acolyte could be saved. However, all the hits are assigned at once, and only by exactly lethal amounts. That means whatever you do try to save is guaranteed to live. Your opponent will have to anticipate a rescue attempt when considering what to hit.
Horrors upgrade : You're allowed to summon, following all the normal rules of summoning except for the timing and cost. That means if you want to summon from the board, you may do so.
Bolt of Change: As I said, this card uses the word "replace," rather than the words "summon," "move," or "place" that are all synonymous in the rules. It will not affect Quicken Decay or Festival of Sinew.
The Blue Scribes: This is a little iffy, so I hope I got it right. I interpret this as adding corruption in a region, giving Tzeentch an extra dial advancement counter during the summoning phase. I cite the synonymity (yes it's a word) of "move" and "place" with summoning. If they're similar there, it seems realistic to think so here. It certainly didn't significantly alter Tzeentch's power for dial in my games one way or the other. However, there is one interaction I had not considered until I wrote this. Tzeentch has no phase restriction for his dial advancement like Khorne and Nurgle (pay close attention to the wording on their character/reference sheets). By that logic, this card could be a real dial mover for Tzeentch. I strongly request feedback on this!
Fall from Grace upgrade : I let it go at the time because it really didn't matter in the game at hand, and I also didn't have the proper reasoning. Slaanesh had 1 cultist, 2 warriors, this upgrade, and Havoc was in the region.
Rise to Glory upgrade : Just as a point of clarification, you don't need a figure in any regions to dominate, nor do you necessarily even need card costs with this thing. Heck, Norsca with 1 Noble and 1 Skaven + Rise to Glory and nothing else in there = 2 points for Slaanesh.
Pleasure Shield: This is up to interpretation, but I don't think it's a large stretch of the imagination to figure that two of these make the equation of 2(2X)=Y. For non-math magicians, it would double twice, meaning the Keeper of Secrets could get up to 16 defense.
The Under Empire upgrade : VERY IMPORTANT DISTINCTION OF MECHANICS! For the first time, we have to realize how ruination goes. You check to see if it's ruined during the corruption phase, and then score the ruined regions in the end phase. Let's say theoretically that 5 regions have Skaven tokens and they're all going up in flames (only 5 regions can be ruined at all). Horned Rat exists in region A, then immediately proceeds to region B that would ruin, and so on until we get to region E. We'll say it's Estalia. All that movement happened during the corruption phase when ruination is checked. That means he's not in regions A-D when scoring ruined regions in the end phase. He'll get the "ruiners" points for regions A-D, making 3+4+4+5=16 points. For region E, which we've settled on Estalia, he gets "ruiners" points for existing there, making 5 points. Then you check to see who has how much corruption. With this much movement, he likely has 1st place for 11 extra points. 16+5+11=32 total points. You do not get the 1st/2nd place points in regions A-D.
Skitterleap: Unlike the Under Empire upgrade, this works during domination. You check for domination before checking for ruination, so if you move away with Skitterleap, you do not get the ruiner's points. However, you will get ruiner's points as well as 1st/2nd place wherever you settle if it goes up that round.
Warp Token Cache: You get the extra 2 power points at the beginning of the summoning phase, so you have to decide if you want it before the first move is made, be it by Khorne, Nurgle, or even Tzeentch (if neither of those are present).
Verminous Horde: It looks like Temporal Stasis, but it's not quite as power. RTFC. This increases costs by 1, not 2.
The Horned One's Due: Look very closely! At first glance, it appears this game came with an extra Horned One's due card. That's true for the most part. However, look closely at the new card with the Morrslieb mark. It's an event card using Event tokens. There's two implications to this. 1) It doesn't add Skaven tokens, so the Horned Rat is kinda screwed with it. 2) It kicks off whatever other Event card is working already.
Rousing the Rabble: There was some discussion as to whether this works only if heroes already exists. To me, this is just foolish. I eventually won the argument just because with 4 hero tokens in the game, it clearly doesn't have much power the way they said. For the record, I rule that all peasants act as heroes. The point of contention was the word "additional."
Closing the Portal: Tzeentch particularly hates this card, but it applies to everyone. Draw 1, not 2 or up to 5/6.
Light of Day: With this card out, it becomes much more realistic that more than 5 regions would end up being corrupted once it leaves. Keep in mind that only 5 can ever be corrupted at all and it all starts with Norsca, following the arrows.
Card interactions
Rampage vs. Greenskins Invade/Electors Sue for Peace/Vermin Outbreak: These are the Old World cards that nullify or minimize battle. Khorne's always hated them. Greenskins Invade makes a maximum of one battle die, Electors makes none at all. For Greenskins, it actually just checks the battle phase, while Electors says there can never be battle dice. Because of this distinction, Rampage gets around Greenskins. Electors still works as normal. Vermin Outbreak also just looks at each battle, so whenever a battle happens, it restricts by two. This similarly applies with the old card Reborn in Blood: 1 for Greenskins, 0 for Electors, 2 dice once + 2 dice in the other battle for Vermin Outbreak.
Mystical Disregard vs. The Horned One's Due (old version) & Empty the Warrens: This actually came up, though we also made some other misses in that game. The Horned One's Due restricts the placement of Chaos cards in regions that have Skaven tokens to 1 instead of 2. A player already had a card in a region free of Skaven tokens, and then the Horned Rat played Empty the Warrens to add a Skaven token. That effectively went beyond the maximum of the Horned One's Due because of the timing, but it worked out. I was the Tzeentch player and played a card on top of another one there (they were both 0 costs with instant effects, so it didn't really matter which). I wasn't playing a second card, just playing a card on top of one already there. I'm pretty sure this is how it would work out, though it was rather awkward when I did.
Fall from Grace vs. Havoc: Yes, this actually happened. Slaanesh had 1 cultist, 2 warriors, and this upgrade while Havoc was in play. He argued that this card makes no statement on phases, so it can stretch across them. I disagreed, but let it go because it really didn't matter for the game in question. The evidence to back up my statement actually isn't where I thought it would be. Khorne and Nurgle reference "the same phase" on theirs, but not Slaanesh & Tzeentch do not (the Horned Rat does not need to). My argument now is that there are two actions that add corruption: the warriors during the battle phase (when Havoc works) and the cultist in the corruption phase. Fall from Grace checks each individual action. Of course this is all up to interpretation, but I feel I'm in the right here. I would strongly request getting some feedback on this !
Shroud of Secrets vs. Rat Ogre upgrade: More a strategic note here than a rules issue. Don't play this with 1 Peasant and your only rival being the Horned Rat. If you can't kill his Rat Ogre, you'll be forced to hit the Peasant, and then you're not protected anymore. Slaanesh has to hit first.
Clan Rats upgrade vs. Grey Seer: As stated in the blog, you can get 4 cultists at once this way. Just FYI though, it's supposed to be "gr a y," not "gr e y."
The Council of Thirteen upgrade vs. the Horned One's Due (both versions): This one is extremely tricky. I understand that the intent of this upgrade is to allow him an extra card where he couldn't normally play it, but the wording is just too sketchy to make that work. The effect of the Old World card (both versions) says that only 1 Chaos card can be played there instead of two. The Skaven/Event token does not actually fill the slot; it just restricts whether something can be played there. That means both slots will not be filled, so the Council of Thirteen will not work.
Overall, it's a very enjoyable set, but more difficult to interpret correctly. I already had a lot of trouble with the first set, making corrections to my rulings even going into this year. That's why I really felt this would be helpful to the community. I appreciate any feedback, and hope this could be stickied for future buyers.