The recent news and discussion about Blizzard buffing monster damage output in the beta got me thinking. I don't want mob-fighting in the open world to be completely trivial, but I don't want it to be too hard either. What do I want then?
To be honest, I don't think I actually want the average mob to be truly challenging, assuming it's of the same level as my character and on its own. I'm not saying that you couldn't make a good game where every single fight pushes you to your limits, but I don't think that WoW should be that game. It wasn't even that way back in Vanilla, back when many other things were generally harder.
Still, killing monsters should cost you some resources at least, so it shouldn't be a case of waltzing in and insta-gibbing things with two dagger stabs. I guess to me personally "a third" sounds like a completely unscientific but decent rule of thumb. If killing a mob takes off a third off your health bar (as a melee class) or costs you a third of your mana bar (as a caster), I'd consider that quite reasonable.
However, at the end of the day I think it's actually a mix of other factors that eventually decides whether I enjoy going out into the world to kill things. For me these factors are: variety of mob types, positioning and dangerousness. Let me illustrate what I mean by example of Redridge Mountains, a zone that I feel does all three of those things pretty well (or at least used to, before they removed all the outdoor elites).
As far as mob types go, Redridge offers a decent amount of variety. Leaving aside the human settlement, you'll encounter three kinds of humanoids: murlocs in and around the lake, gnolls in the hills and Blackrock orcs spilling forth from the Burning Steppes in the north. In addition there's a solid amount of wildlife: boars, black dragon whelps, condors, tarantulas and threshers in the lake. There are even some undead, demon hounds, a rare giant and a rare elemental spawn. The differences between these mobs aren't purely cosmetic either. Some of them cast spells, some frenzy, some poison, some stun and so on. This guarantees some variety as you quest your way through the zone, and different mobs encourage you to use different abilities based on what class you play.
It should be a bit of a no-brainer that this is something that makes the game more fun, but somehow this hasn't prevented Blizzard from making zones where a single type of mob dominates so strongly that it gets tedious rather quickly. For example silithid aren't strictly the only type of mob in Silithus, but considering how much of the map is taken up by their hives it often feels that way, and it makes the area a very boring place to quest in.
Then there's positioning. As a general rule, mobs in WoW are positioned in a very "gamey" way to make it convenient to pull them one by one, while leaving as little map space as possible completely empty. That's always pretty unrealistic of course, but to a certain extent willing suspension of disbelief works around it. To use the Redridge example again, gnolls tend to band together in camps, where groups of them sit close together around campfires, and others patrol around the edges of the camp. This makes at least some sense, and provides some interesting opportunities to practice proper pulling and crowd control. Can I get this patrolling guy without aggroing any other mobs? How can I get past this group of three without dying?
In other parts of the world however, especially in the more barren areas, you'll often look out across the land and the mobs will look very much artifically arranged to stand at a certain distance from each other, often not even moving much. Again, that makes for pretty boring gameplay regardless of how difficult the mobs are to beat on their own, as you can do little but simply pull one after the other.
And finally, there's the sense of dangerousness. This is the aspect where variety is more important than anywhere else. To use Redridge as an example again: when you're questing there (as Alliance), the town of Lakeshire is obviously the ultimate "safe space". But the world outside isn't equally dangerous everywhere, regardless of the mobs' levels. For example there are a lot of neutral boars in the immediate vicinity of the town and near the roads, mingling with the hostile mobs there and lowering the average danger per sqare mile so to speak. With neutral mobs among the hostile ones you're less likely to overpull by accident, and if you suddenly need to go AFK for a minute it's easier to find a reasonably safe spot to stand. One step up is the area around Alther's Mill, where all the mobs are hostile but still somewhat spread out. The densly populated gnoll and orc camps are a lot more dangerous already, but the ultimate danger used to lie in Stonewatch Keep, back when all the mobs inside it were elite. You just knew that this wasn't a place where you wanted to go alone.
Different areas threatening you with different degrees of danger keeps you on your toes and makes the game more intersting and immersive. The reason variety is so important here is that veering too far into either direction has a negative effect as well. If you make everything neutral and unthreatening, like Blizzard has done in the starting zones a few patches ago, the game leaves you bored and bewildered (because those "evil" quillboar not minding your presence makes no sense). If you make every monster really dangerous on the other hand, you discourage solo play, which isn't really what WoW is about either. I remember old Silithus being a nightmare before they removed the outdoor elites, simply because all the silithid were elite, and with a couple of additional elite monsters and elite Twilight cultists the whole zone just felt like a place where you couldn't do much on your own. I quested there with a friend for a few days and then never came back, not even on my alts. It was just too off-putting.
Assuming that a zone presents the player with different types of mobs; some grouped and some alone; some neutral, some hostile and some really hard; I don't think Blizzard needs to turn every individual mob into a hard hitter. As long as the mobs are not completely trivial, easy and challenging areas can be implemented simply by adjusting the aforementioned factors.