Pick your favorite game with mobs of various levels, and pick an arbitrary level 20 mob in that game. Not a boss or one with something special about it, but just a typical level 20 mob. Is that mob difficult?
The question, of course, doesn’t make a bit of sense. For a level 1 player, the mob will likely be unbeatable, at least solo. For a level 50 player, defeating the same mob would most likely be completely trivial. It thus does not make sense to speak of the mob as being easy or hard, as this varies too wildly by level. All that one can do is to say that the mob is a reasonable challenge to a player of this or that particular level.
There is nothing peculiar about level 20 here. The same could be said of mobs of most other levels. The handful of mobs that are easy to kill solo at level 1 with starter gear could perhaps be said to be easy, period. At the other extreme, mobs that would be difficult to kill even at the level cap with the best possible gear, in a group (or raid) of the maximum size allowed, with the ideal class distribution and all of the rest of the players comparably leveled and equipped could perhaps be said to be hard. That is, if the mob is difficult no matter what you have in your group, then we’ll say that it’s a hard mob.
But hard mobs scarcely exist in MMORPGs. PvE content is meant to be beaten. If something in a game is hard, some players will get stuck on it, and be unable to move on. Those players will most likely quit the game, and hence stop paying the monthly fee. If there is quite a bit of content after the hard mobs, that’s content that the players will never see before they quit, and months of potential monthly fees that will go unpaid. For business reasons, a company cannot allow this.
There are perhaps a small handful of genuinely hard mobs here and there. These generally don’t gate off content beyond them. Sometimes they’re meant not to be beaten, such as the Black Ship from Puzzle Pirates. But these are necessarily scarce enough to not add up to a difficult game.
Let’s try another question. Is it harder to kill ten furbolgs, or to kill a thousand furbolgs?
The obvious answer to that is that it is harder to kill a thousand. Indeed, this entails killing ten, so it certainly cannot be easier.
That is not quite right, either. How many players are there who can kill ten furbolgs, but could not kill a thousand if so inclined? A player who has killed ten furbolgs has repeatedly demonstrated that he can kill furbolgs. There is rarely any reason why he could not repeat this enough times to kill a thousand if he cared to. Indeed, since he may level in the process of killing furbolgs, and certainly learns something about their spawn locations and AI, the last ten furbolgs are probably easier to kill than the first ten.
Someone who would claim that it is harder to kill a thousand furbolgs than to kill ten probably meant that it takes longer. And so it does. But to say that something takes a long time is a very poor substitute for difficulty. That would be to argue that anything that takes a long time is difficult, a conclusion which is completely absurd.
For example, in Guild Wars, is it difficult to be awarded a third birthday present by the game? It does, after all, take three years, and there is no way to speed up the process. Surely it cannot be considered difficult to do something that consists only of creating a character and then waiting three years.
Single player games don’t have this problem. If there are ten levels in a game and you get stuck on the seventh, the company doesn’t particularly care that you never see the last three. You’re not paying subscription fees, anyway. Not that many players ever beat Who Framed Roger Rabbit because Judge Doom was so hard, but from a company’s perspective, that was just fine.
Still, if an MMORPG wishes to make a game challenging, they have to create ways to get around the challenge. If the ways to get around the challenge are too easy, players will just use them most of the time and skip the challenge--that is, players will skip most of the game. See, for example, how commonly some players like to tag along while much higher levels kill everything, or take larger groups than intended.
The key is to make it so that taking on the challenge in the proper manner levels you a lot faster than trying to avoid the challenge somehow. For example, if you’re level 31 and try the level 31 content and beat it in the intended manner, that makes you level 32 immediately. If a good player takes an hour to beat the content, and someone else could have spent an hour grinding mobs to get from level 31 to level 32, then a lot of players will do that and skip the challenge.
If grinding mobs without attempting the challenge makes it take ten hours to get from level 31 to 32, then that would push players to try the challenge in the intended manner. Still, that would provide a way out so that people don’t get absolutely stuck in one area forever. Try a dungeon, get reasonably far into it, and then fail, and you still get some experience. Do that a dozen times and you get the level without having to beat the dungeon, so that you can move on if you wish.
The common exploits of bringing a high level player or more players than intended would also have to be shut down, to avoid letting players level fast by these methods. I don’t know of a way to cap the number of players apart from instancing, but that has been used in a lot of games and works pretty well. The automatic level could be awarded only if your group doesn’t have anyone over the intended level for the content. A sidekick system somewhat comparable to what City of Heroes uses could allow higher level players to help their lower level friends without making the content completely trivial.
Thus, it would be possible to make PvE content in an MMORPG that is genuinely challenging. Stupid hassles like grinding timesinks are a poor substitute for a genuine challenge.