Leveling a character in World of Warcraft is normally not very challenging. Unless you make up your own rules that MAKE it challenging: Like the WoW Ironman Challenge, in which you not only have to reach the level cap without dying, but have to do it using only grey or white items, no talents, no external buffs, no nothing. Of course the disadvantage of self-made rules is that they are often not very balanced, and in the case of the WoW Ironman challenge it turns out that the challenge is only possible to do for hunters. Nevertheless the idea is a nice one.
World of Warcraft supports such alternative rulesets indirectly, by having the armory in which rules compliance can be checked. It would be a lot harder to run such a challenge in another game, where you would have to trust people's word that they never died or never used forbidden boosts.