So I was in western plaguelands over this past weekend on my hunter, and in between getting slaughtered when my pet would run up and chain aggro nine other mobs and having the mobs I did manage to single pull randomly evade bugging and running back to their silos, I realized that I was still enjoying the hell out of that zone. Going into WPL and especially Andorhal felt like I imagine going into a real undead infested city would feel, even though this occasionally makes for extremely frustrating questing and solo play.
This, and QQ threads about pvp and pve balance on the forums, got me thinking about the conflict between the game needs and the world needs. Like in the example of WPL, a undead infested city is a cool world concept that fits in well with the scourge, but from a gameplay point of view can be extremely frustrating. And on the other end of the spectrum, spread out and isolated undead are a much more managable solo experience, but really takes away the feeling of a land that is completely controlled by the undead, which I believe is an important part of the experience. TBC pushed pretty far in the direction of gameplay trumping world design, but WotLK hit close to the proper balance, in my opinion. Especially in some of the minor details, like having mountain rams randomly sitting in out of the way places, or having the frost protodrakes fly down and kill rhinos, then pick up the corpse and fly away. Small things which have no real impact on the gameplay, but sort of help prop up the illusion of a real, continuous world. The little scarlet groups in WPL who would go to the farms and andorhal and fight the undead are another good example of this. I always liked those, even though they usually created a small radius that was even more obnoxious than just the undead huffing about.
And it's not just in world design, but also in creature AI and dungeon layout. It reminds me of that old Flintlocke joke, from when they're infiltrating the Scarlet Monastary. First, when the sentries find the corpses, one says, "Look, dead bodies!" and the other says, "OMG Where?!" Because patrols who spotted and then immediately went looking for the people who killed these bodies, while more realistic, puts a huge damper on the party who's trying to get mana up after that trash pack. And once that patrol alerted the high commander, he called for the Impenatrable Scarlet Defense, which meant having all the crusade soldiers going forth and standing in small groups of 1 or 2, waiting to be pulled, instead of rushing to the door to deal with the intruders. Because while realistic, having the entire zone aggro and rush you as soon as you let a runner get loose is absolutely no fun for a game like Warcraft. Dungeon design and npc AI is designed to meet in the middle between the realistic version and the fun version. If you let a runner go free, he's going to bring some friends back, but he won't raise the alarm and bring the whole zone down on you.
And this brings me to PvP versus PvE balance. I've seen a lot of posts that say that blizzard should just have PvP mechanics and PvE mechanics and completely unlink the two, allowing for both sides to be balanced independently. However, this completely destroys any concept of a continuous world. They already have some fairly severe departures, especially in terms of CC duration, but those fall under necessary to have a balanced game (30 second polymorph is important for dungeon CC, but completely not-fun in a PvP setting). The biggest clunky mechanic was exorcism, where they had to make it simply uncastable on player targets to reign in ret burst, until they could figure out a better way to do it. Having PvP and PvE follow mostly the same rules really helps keep some simplicity in the game and maintain the continuity of the world, since the horde and alliance conflicts are a big part of it.
Ultimately, this is why lore is so important to the game. This is something that was largely brought into focus with the T9 raid, and the issue with it's lore. Primarily, that there should be some. Yeah, there's the tournament there, and there's the prep for Icecrown hovering in there, but what you really have is 5 encounters that are just floating in space, with no real lore to hold them together, in contrast to Ulduar, where there was so much lore they had to actually make an official page on the WoW website to explain some of it. Make the tournament a training area instead, prepping crusaders for the attack on icecrown citadel, and change the raid into some sort of first battle in the final campaign against Arthas. Throw in a slightly simplified version of Hyjal Trash, with only one or two waves per boss, and turn the first three encounters into scourge-related counterparts: three powerful undead monsters, a master necromancer who summons ghouls and stuff instead of demons, and a team of the scourge's most poweful lieutenants, each who happens to resemble one of the player classes. Then the twin valkeryie and Anub can be left as is, and Arthas will at least seem to still be a threat, instead of letting us capture some of his most powerful minions and having a huge party tournament right on his doorstep, complete with minstrels and fried food.