It depends on how exactly the "difficulty" is generated, but in any one game the difficulty should be tied to what it is you are tackling & the complexity tied to how many people are being expected to deal with the situation.

A solo-quest mob can be tricky up to a point in what can & cannot be expected of the average solo player, a group can tackle a trickier mob that has more abilities up it's sleeve, & raid mobs should have multiple ways of being tackled.

The trick though is not sticking to any one formula, racial "habits" for creatures is a good way to let players have some expectation of the creature they are facing's abilities & behaviours & gives them some forwarningof what to expect, though with a surprise here & there if it's more powerful than the average type.

Variety of difficulty "types" & ways to win are what lend polish to any encounter though, from a single Kobold, to some mighty overlord with hundreds of underlings, environment should play a part too in more complex situations, keeping players on their toes but also making it so every single player involved can feel that they made a important contibution to the overall sucess.

So, I can't give a "simple" answer to the question, I like variety, some things should be relatively easy, others, such as the end boss of a high level raid dungeon should provide a huge challenge in terms of what it takes to win & be a combination of having farmed prior content & learnt prior bosses strategies & dealing with several things happening at a time & progressively.

What I did like about WAR's dungeons was how it felt like a raid for 6 people (particularly Lost Vale), & that was a shining point of it's PvE content, group content like that with it's own progression was very entertaining, though some didn't like it & felt it was an artificial time-sink taken in the proper context it was very well done.