I don't think they have. The fact that it took longer to level isn't much of a challenge, it just, well, takes longer. As for group quests, the ones that are there and the ones I did I wouldn't call hard either. As far as I can remember they had 2 modes; impossible on your own, piss-easy with some folks. When leveling my Warrior I try to do them on my own, in that case it is a challenge.
I love the game, content and gameplay-wise it's so much better now, if anything I find it more challenging now thanks to the relatively recent inclusion of certain challenges in the form of achievements, a good way to give the player more choice, to make the fight/game as hard as they want. I like that, giving people freedom to play the game as they see fit.
I guess you're right about the multiplayer part of it though. It's so easy to just group up with randoms and it's actually more rewarding than finding a solid group to play with on your own server, a group you can bond with and adventure with. I wish they would reward that a lot more, promote that a lot more.