Well the thing with WOW is that it hit the perfect point of casual and hardcore.
You could solo, which most MMOs at the time didn't let you do, but if you did it in a group it would go faster, and you'd have more fun, and naturally meet other people.
But WOW was developed by Tigole, every good design WOW had was designed by the rest of the WOW team, the 1-60 experience was on point, but Tigole insisted that WOW was about raiding, WOW was about end game, even though their own numbers shown that only 10% of the playerbase ever hit end game by the time BC hit, and of those 10% only 30k players were raiding on a weekly basis, of their 4-5m preTBC player count, that would mean less then 1% of the playerbase actually raided.
And people loved WOW, it was a great MMO starting out, but once you got to end game, it was raid or die, and that has been the most persistant issue WOWfags have had with WOW in general, that unless you raid, there was nothing to do at end game, and this issue was never ever addressed, the only way blizzard was able to 'fix' it was to add in LFR.
But again, after launch WOW became "raid or die" end game focused, and because of this any and all content had to cater to the raid community, thats why in TBC, they made raid sizes smaller so they're easier to run, they made end game dungeons an absolute joke loot hallways where everyone just ran Mechanar for badges, and anything outside of raiding was pretty much dead content.
And they nerfed exp rates completely, but at least Karazhan was fun enough to run the entire expansion long for most casual players and PVP gave out ezmode gear to keep people logging in weekly.