The common complaint from my friends is no interesting/challenging solo content. They're well-aware of what WoW is and what MMOs are all about; they're just dissipointed that they either raid on schedule with a guild or PUG.
I wish Bliz would hurry up with a Timeless Isle 2.0 -- I feel like that's the answer we need for a lot of people.
Timeless isle was boring for me, but I guess some people liked it. It had a bit of a lottery feeling with chests/nameds for 496 alt-friendly upgrades and it wasn't trivial mindless grinding, so it had something worth mentioning.
They did several mistakes in WoD (in my opinion): remove any form of challenge whatsoever from old raids, so instead of providing alternative content for soloers or small groups of friends, they made everything zergable up to and including cataclysm stuff. Making heroic dungeons pointless so easily and so quickly, the fact that my slaves in the garrison can bring me better gear than challenge mode runs give when completed, the segregation of the population in solo instances (garrisons), the removal of scenarios MoP style for multidps groups without tanks and healers, several decisions in game design that are frankly horrible (let's say at least debatable in several examples) such as healers mana management, in some cases excessive pruning of skills, neutering some mechanics that made certain classes more interesting to play (atonement) and so on and so forth.
Interesting and challenging solo content doesn't exist (anymore) and cannot be implemented, because the vast and I mean vast majority of the population can't solo its way out of a wet paper bag. I used to be an amazing solo hunter (self esteem inflation, sorry for this), even if several people in the world were way better than me, I nonetheless scored a couple kills (at level 80 most of them) that few people could do, for example Kael'Thas at 80 and Yogg'saron at 85 (never got Alone in the Dark solo tho, I think only a couple hunters made that). In the end nobody really cared, because it was working around old raids mechanics and loot was basically some gold and useless items, also it was so frustrating to learn the hows and whens that having the patience to do it over and over wasn't the cup of tea that could be served to the general population of an MMO and sell it as solo content.
Add to this that some classes are just solo gods, while others are pretty much shit. Without disturbing exceptional players like Mione, I bet that any of you can solo shit a lot easier on a blood dk rather than on a mage or a priest, am I right?
A good metric was the green fire quest in MoP: my lock has the title Of the Harvest and it took me a lot of tries (I mean a lot, I think I sunk more than 2000 gold in repair costs, probably over 60 tries), because I'm really not that good at warlock, certainly not an expert, but in the end I scored a kill with a shitty item level (496 or thereabout if I recall correctly). There were so many folks that couldn't do it in full 528 SoO raid finder gear and still a bunch of warlocks that couldn't do it even in 550! I did it again in 540ish and it was very easy, but I already had the necessary experience by then.
So yeah, I don't see solo and challenging content being a thing, ever, at least if they want to reward it with current items. They could technically rework that shitty damage buff on old raids and leave those as a source of fun for a bunch of people, but I guess it's not happening anytime soon.
For Blizzard what was in the past, stays in the past, until they need fast content and revamp and old zone.