There has been a lot of discussion since Wrath of the Lich King came out whether World of Warcraft has become too easy, or whether making it easier and thus more accessible is a good thing. But these discussions have been mainly concentrated on the endgame, in particular the raiding endgame. The reason for that is that challenge and difficulty are better defined at the level cap. As by definition your character doesn't get any stronger any more through the gain of levels once he is at the level cap, the only variation of power is through gear. With some reasonable assumptions of what the character might have done just before reaching level 80, and what gear he might have acquired, we end up with a pretty good idea of what stats a freshly minted level 80 character has. And then the discussion of how challenging it is to do the very first raid dungeon with that sort of stats, or inversely, how much more gear somebody should be required to gather before entering the first raid dungeon can begin.
But patch 3.0 and Wrath of the Lich King not only changed the difficulty of the endgame. They made characters stronger over the whole level range. And, specifically for lower level characters, they introduced a whole new way of twinking (using your high level character to procure gear for your low level characters) in the form of heirloom items. Also the amount of experience points needed to level up has undergone several changes since WoW 1.0, so leveling up is now faster than it ever was.
Now normally the question of how difficult or challenging the lower level game is shouldn't pose itself, because the player has better options to choose what level of difficulty he wants to tackle. If the content at his level is too easy, he can simply fight monsters of higher levels, until he finds something challenging enough. Or can he?
It turns out, as so often, that players are not interested in a challenge per se, they are interested in bigger challenges that give bigger rewards. That is not only true of people who dismiss endgame heroic raid achievements are gimmicks, and aren't willing to fight with one hand tied behind their back for higher challenge, because there is no real reward for it. The same principle applies to lower levels. And it turns out that in World of Warcraft, fighting higher level monsters isn't really well rewarded.
The most extreme example of that is very visible to me with my level 70 mage. I have been visiting Dragonblight with that character, because the location to make Ebonweave has been moved there. I know by experience that I am able to kill the monsters in that zone with the spells and gear I have. But doing so would net me *less* experience points per hour than staying in Howling Fjord or Borean Tundra. Not only does killing a higher level mob give only a fraction more experience points per kill than killing a lower level mob. But more importantly at level 70 you don't get any quests in Dragonblight, they all have a minimum level requirement of level 71, despite being level 72 quests. As quest xp easily make half of the total xp you gain by playing, doing lower level quests in Howling Fjord / Borean Tundra gives more xp than just farming higher level monsters in Dragonblight. Thus I'm actively discouraged of seeking out the highest possible challenge I could handle, because I'm much better rewarded if I just stick to the easy stuff.
Besides my level 70 mage, I'm also leveling up a druid from scratch, now level 18. There is a certain fun to be had from the process of twinking. I just got my second heirloom item for the druid, going for the Dignified Headmaster's Charge staff after previously having bought the shoulders. As my raiding priest doesn't have anything he could spent emblems of heroism for (for himself), I'll probably buy the heirloom trinket next, once I got the emblems together. In addition to the excellent bind to account stuff, the druid is equipped with the best armor money can buy at that level. And my various crafters provide him with all the consumables (potions, food buffs, even scrolls) he could need. All that makes the druid significantly stronger than the last druid I played on the US servers in 2004, when WoW wasn't out in Europe yet (I switched to European servers when it came out there in 2005).
While being twinked and overpowered can be fun, I do have trouble finding content that is challenging enough. There is less of a problem with minimum levels of quests being too high. But the whole "flow" of quests guiding you through the various zones has been designed for characters who are weaker and level less fast. Previously by the time you were finished with a zone, you got a quest leading you elsewhere. Now you need to skip a lot of quests, otherwise you get stuck in far too easy green quests.
Of course when WoW came out, and everyone was low level, the real challenge was doing dungeons. Well, the dungeons are still there, but it is nearly impossible to gather a group for which the dungeon is a challenge. There simply aren't all that many low level characters around on the old servers. And those low-levels that are around often prefer to be "boosted" by some high-level character, be it a friend or by dual-boxing. And even those boosts are somewhat a waste of time: With the heirloom and bought items you already have, and given the speed at which you already level, spending time in a dungeon for better gear really isn't necessary.
So leveling my alts isn't as much fun as it could be, because it isn't as easy to find a challenge, except randomly charging into higher level monsters without having even a quest for them. As a twink it is hard to find a meaningful challenge giving a meaningful reward. While in a single-player game you might want to play at a lower difficulty setting the first time you play it, and then crank it up for the repeats, World of Warcraft doesn't have a simple switch to make the game more difficult. Due to changes in the game the alts now have it already much easier than the original characters we played when the game came out. And unless you have an iron will and force yourself not to twink your alts at all, the whole alt experience risks becoming so trivially easy that it isn't much fun any more. Especially if you are going through zones and quests you already know. No wonder so many people prefer to skip the first 54 levels and start with a hero class death knight right away.
I'm starting to wonder whether instead of making the low-level content so trivial, Blizzard shouldn't provide a different way to make alts. Either by simply giving other classes the same option as death knights, to start directly at level 55. Or by some way that requires some action from the higher level character, for example the ability to "buy" levels for alts up to a max of 70 using emblems of heroism or another currency. Hey, what about all those achievement points we collect on our high level characters, how about you let us buy levels for alts with those?