At no particular point there is any considerable challenge presented to the player, not even the 15-50 instances provide any meaningful challenge to the groups at that level due to the fact that the players mostly overgear the content. I've also noticed that there are usually one or two characters in a random group at the 45-55 range (at least) who are either a couple of levels higher or lower than the rest. This in turn means that the ones lower get carried through and the ones higher get to carry the rest.
There are challenges in WoW later on, and a quite huge amount of them, really. Let's list some of them for discussion:
- Run only LFD PUGs and not lose your temper at all.
- Endure a jerk in a random LFD group.
- Gear up after getting into level cap.
- Gear up for raiding, even though it is now possible both by doing heroics AND normals.
- Find the right strategy guide and accompanying videos to heroics and raid instances.
- Learn the instances by heart before committing to them, see 4.
- Learn the raids by heart before committing to them, see 4.
- Play some minigame while waiting for a random group and beating yourself every time, unless you are blessed with a guild running at the same level and progression.
- Do not leave a group in an instance in which no-one understands the basics of crowd control at all. This applies to the instances from Dire Maul onwards, really, only to be forgotten in Outlands.
- Level outside the pre-planned storyline.
- Level without killing anything.
As you can see, learning to play your class isn't included. Naturally. That is not a challenge, that means only that by going through the quests and questlines you should have learned everything there is to learn. At level cap you should be a master already, even though it takes 10.000 hours to master anything.
We all know it doesn't take 10 000 hours to level from 1 to 85 anymore, though...
Anything else I forgot to mention as a challenging thing in WoW?
PS. I know a certain German reader who will point out that 'to have fun' would be one, but I doubt I wouldn't be playing the game still if I didn't have at least some kind of fun with it, right?