I think the thing is, with MMO's the strength of an MMo is teamplay. And people are generally buttplugs so not worth teaming with. So it's frustrating finding 6 random people that get along well enough to make the most out of the experience.
EQ, for all it's multitude of flaws, originally seemed to grasp this when Verant ran it. A good example is the game pace was slow, and way too simple to be entertaining. Downtime was a *****. But this gave you time to type communications to your team and socialise a little, and it was THIS side of it I actually enjoyed, because I met some good people through it. Soon, I found the guild socialisation was the main thing that I enjoyed. (And I do have a life outside MMO's. I was at uni, playing sport and hanging with mates often enough.), and at higher levels when people needed to be co-ordinated, that slow pace helped.
Also, no auto maps and vision problems based on race so just getting about could be tricky and fun, even if you where high level. Some stuff that seemed so frustrating added challenge that is sorely missing from WoW and others. Good example is being a Human level 20, it was easy to get about the lowbie woods just next to the halfling town. But when night fell, no vision and Level 40+ mobs made it a veritable no go zone unless you were feeling pretty sure. And the long corpse run with no gear made it a real consideration. I ran the gauntlet heaps of times because hugging the boundary was a pretty good chance of clean crossing. Though I did get caught out once and nearly caught more than a few times by mobs on the boundary.
It had a lot of things that in retrospect, made the game a challenge where other games let you have a free lunch. Crossing the mainland as a level 8 is doable, and quite hard if you're just starting the game and want to get to your mates starting zone and you know no one, and have nothing. But when I did it, I proclaimed booyah!
Also, the levels had some atmosphere. And being stuck in first person mode because third person was almost unusable made it more immersive. Stupid but true. Lock in first person to help make wandering sentries more of a challenge rather than something dead obvious. You don't want to turn from the fight, but you don't want something sneaking on your back either.
So challenge really, is what is lacking from a lot of games. But what isn't needed from EQ? Buggy programming, pathing and Ai. poor Ai, fetch/murder quests from here to hades and back, balance and economy issues, predictable combat in normal situations, levels and gear being the 90% requirement to win a straight fight, as opposed to skill. There needs to be some reward for gear and character experience, but skill needs to be more like 50%. (Mind, skillful druids and chanters can make a difference. But warriors? So long as they taunted and hit stuff, they are gear driven.)
Gotta go, but some challenge to the world at large by promotion of good social skills rather than just the fights can help an MMO be different to a single playr game. That's where I find WoW falls down. I never really found it utilised socialisation much.