Which is, I think, the point @
Faroth was making. Most MMO's only get close to being difficult at a point most players are never going to reach. It gives the impression that the game world lacks any kind of real danger. It's right to an extent too, for the most part you're in very little danger of death in the open world in a Themepark MMO unless you venture to an area well above your level range.
I was perfectly comfortable fighting mobs in groups of 3-4 in FF 14 while leveling without any major issues. Leveling as a Prot Paladin in WoW ment I could take on groups of 15+ mobs at times. Lack of danger outside of high level content is a very common theme in MMO's.
It's not a problem that is beyond fixing however, but providing a challenging open world experience for all your players is extremely difficult for a Hotkey based MMO to pull off. Especially when your players come from all kinds of gaming backgrounds and skill levels to begin with.