The short answer: Most of PvE got easier, while a few sections got harder.
The long answer:
As everyone who wasn't living under a rock knows by now, the 1/5/2012 update reduced monster armor and increased monster health.
From a player-side, skill-by-skill perspective, what happened was that:
(1) Armor-ignoring skills all got weaker, since they have to chew through a bigger pile of health with the same DPS; and
(2) Armor-respecting skills seem to have gotten stronger, since they do more DPS now, even though they have to chew through a bigger pile of health. (I say "seem" because we don't have the precise numbers on either change, but observations indicate that armor-respecting damage is killing things faster than before.)
Obviously, this encourages player builds to shift towards armor-respecting damage. (Or, to be more exact, good player builds have shifted/are shifting from heavily focused on armor-ignoring damage with a little bit of armor-respecting damage mixed in (mostly in the form of base damage) to a mixture of armor-ignoring and armor-respecting damage that uses the most potent skills of each. Bad player builds have always mixed damage types without being aware that it mattered, and they continue to do so.)
A slightly broader monster-by-monster view is more revealing:
For purposes of looking at the impact of this update on PvE difficulty, we can divide monsters into three classes.
Class 1: Monsters with no effective ability to resist armor-respecting damage.
The vast majority of monsters in GW had just one method of resisting armor-respecting damage -- inherently high armor -- and now they have none. These monsters die faster now because anything the player throws at them is going to work. (Even builds that are heavily focused on armor-ignoring damage usually contain enough collateral armor-respecting damage that they seem to come out ahead vs. class 1 monsters.) So, most of PvE got easier.
That's the main outcome. The remainder of this post is looking at the exceptions to this general rule.
(Less importantly, monsters also became more vulnerable to unfocused/collateral damage. Previously, if there were a couple of bone horrors off in the corner hitting a monster (or Koss, who's about as smart as a bone horror, etc.), they weren't hardly going to scratch it until someone got around to putting Barbs on the monster (or OoU/EBSoH on the horrors). Now, the minions actually hurt things. The same goes for misguided hero auto-attacks, wand shots, secondary damage on utility skills, and many other things that just weren't part of the team build's "directed damage engine." They used to not matter at all; now they matter a little.)
Class 2: Monsters with some effective ability to resist armor-respecting damage.
These are monsters that have some way to resist your armor-respecting damage. It could be that they still have a high inherent AL, or it could be that they use skills to boost armor, block, etc. The impact is this: The armor-respecting damage that players are now packing works poorly or not at all since it's resisted; but armor-ignoring damage also works less well than before since it's running face-first into the hp buff; and, to make matters worse, player builds have less armor-ignoring damage to bring to bear since they changed their builds.
An example might help: How do you deal with a monster using Kinetic Armor (assuming you cannot interrupt or strip it)? Old builds didn't care about Kinetic Armor because you were killing it with, say, Barbs+Orders+SoH. New builds would like to kill it with, say, Double Dragon, but can't. Assuming your build still has Barbs+Orders+SoH, you can kill it that way, but it will take longer because of the higher hp. Of course, your build probably doesn't still have all three of those, since you needed to make space for Double Dragon.
The overall result is that class 2 monsters take longer to kill than before.
Class 3: Class 2 monsters backed by effective healing.
This is where the stuff hits the fan.
The root of the problem is that the added second or two of survivability these guys have means an added second or two for the monster healer to get its act together and throw a heal, which it usually manages to do. So we've gone from spiking out a monster faster than it could be healed (or healed more than once) to trying to out-pressure a healer with such a ridiculously high attribute spec that it can refill the whole bar (or come close) in a single cast.
Example: Go into Slaver's, find a Wretched Wolf mob with a Modnir Priest and a Stone Summit Defender, now try to kill a single Wretched Wolf just by pounding on it. It used to be pretty easy; now it it simply can't be done. (Eventually you'll decide to disrupt the healers, overwhelm them with AoE, or strip the Wolf -- but that's not the point. The point is that the Wolf is now impossible to spike and the pressure battle is now impossible to win, unless you do one of those things.)
The end result is that class 3 monsters are a lot harder than they used to be. That's not to say that there aren't ways to compensate, but the mere fact that you must compensate should be evidence that they've gotten harder.
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One other difficulty-related issue arising out of this update is that certain skills have been buffed or nerfed for monster use.
The obvious buffs/nerfs were anticipated before the update: sacrifice skills (nerfed), comparative health skills (victorious buffed, envy nerfed), PS-like skills (nefed), and health threshhold conditional effects (generally buffed).
The most significant buffs are relatively non-obvious:
(1) Monster healing is globally buffed to the extent that it used to overheal and now it doesn't so much. (Monsters are finally getting some use out of their last 3 or 4 levels of Healing Prayers/Resto.)
(2) For class 2 and 3 monsters, rezzing is buffed since they come back up with more hp and are harder to knock back down quickly and easily.
One other thing I noticed late is that Spirit Bond was buffed.
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Postscript: Slavers.
Xeno asked why I said (elsewhere) that I think Slavers is harder now than before. Basically, it's a perfect example of class 3 monster mobs.
To put the challenge briefly: The Stone Summit Defenders can stall your offense better than they could before (particularly with Shield of Regen), and that buys enough time for the Priest (Summit or Modniir) to slap what is basically a full heal on the target.
A more complete list of skills down there that are now a lot more problematic than before:Wretched Wolf - Feigned Neutrality, Dark Escape (very hard to finish off, survives long enough to get healed) Stone Summit Defender - Shield of Regen (makes it hard to finish off whatever you're trying to kill, makes it survive long enough to get healed), Guardian, (same but less so), Spirit Bond (triggers more often) everything overheals less in general Modniir Priest - everything overheals less, WoH is particularly buffed in this respect Stone Summit Priest - everything overheals less Stone Summit Warder - Ward Against Melee (makes foes somewhat harder to finish off, live long enough to get healed) Stone Summit Dominator/Stone Summit Blasphemer - Renew Life (50% hp is more than before.) Stone Summit Dreamer - Resurrection Chant (dreamer's current hp is more than before) Stone Summit Cleaver - Signet of Return (rezzed allies pop back up with more hp) Stone Summit Summoner - Taste of Death (overheals less, somewhat offset by BotM costing a lot more hp) Slavers is by no means impossible; it's just harder than it used to be and needs some different solutions than a lot of things that used to work.
