TL/DR: Its too easy now.
Back in the day getting to 60 was a challenge but by the end of it you understood the game, knew your way around the world, had covered a lot of lore content, had plenty practice working with others, in your class role, in 5 mans. At any time you had about 40 quests available across both continents, of which you could probably do about 15 and the rest would need to wait to next level. You had to travel to level, exploring the world, finding things as you go. Getting to 60 was in itself a challenge, something to be mastered.
Then when you got to 60 and completed the attunements it was still interesting. I did 40 man raid stuff with the top Horde guild on the server and it was a challenge. Not just for the gameplay but the whole social thing and dealing with such a large group of people. Then Blizzard started the switch to 20 man raids which caused a lot of friction as we couldn't consistently run two raid groups since on different nights different people were on and we often ended up with neither group having the right numbers due to people being tied to the other instance. Drama ensued. People left, splinters formed.
I left the guild to try an alliance character and different class on a different realm I got to 60 about when BC came along and all our work was pointless. People got to 70 over a very long drawn out time leaving the players with less time behind while most of the guild were running the new 20 man stuff. Drama ensued. People left, splinters formed.
I got sick of the drama and quit WoW with some of my friends and went to Eve Online when it was young.
Recently I had some time so I decided to see how the game had changed. I upgraded my account all the way to MoP, started a Monk, Forwarded 500g from my old char to my new one since gold was always short for new characters last time I played, and expected a challenge. What I got was hand holding and no challenge, There was no running around looking for quests, every quest target was close to the quest giver, there was no real way to deviate from the set quest path, I didn't see over half of the old world even by the time I got to 60. The only thing that was interesting was the way the world had changed with as a result of the old content being in the past (such as how the Wailing Caverns storyline caused so much overgrowth and corruption) I kept wanting to skip forward as quests were too easy but this wasn't possible due to the way every quest was following another. Apart from initially buying 16 slot bags the gold transfer was pointless, by level 10 my character was already in surplus. I was glad that when I got to 58 that I could switch to BC as there might be mobs that I don't two hit but that didn't last long, then the same again at 70 and again at 80. As such I never got to any of the various expansion 'conclusions' I have no idea how any of the stories from the older expansions actually finished. I still don't know why Helscream is in charge and Thrall is wandering the world. Towards the end every quest reward was just that little bit better for my current spec which is great for leveling but now I want to try Healing but I have almost no good gear for that. Before level 90 I never set foot in a dungeon or scenario. I've tried a few guilds but unlike the old times there is no 'top raiding guilds' but just a huge mass of guilds that do varying amounts of raiding and finding the right guild for my playstyle/ability/timespend has been near impossible. LFR is chaos and unpredictable and unchallenging as there is no try again, refine tactics, improve gear as a group, try again loop that was something to work toward and ultimately no sense of achievement for killing a boss.
Then there is the other stuff Blizzard added. The farmville clone, The pokemon clone, The daily quest grinds... none of these are a challenge in any shape or form and are just 'something to do' but don't have any real sense of reward at the end due to the ultimate result being down to how many times you repeat the same simple set of tasks and completely unrelated to my skill or my guilds skill at playing the game.