On the contrary... its not a challenge getting to the high level granted... after that is when the game really starts. Only you dont spend a month starting the game (assuming of course you play those 4-5 hours a day).
And really... tell me wheres the challenge in grinding or doing simpleton quests? And wheres the challenge of the PvP ive seen in WoW where people basically are spamming skills and hope they have a better skillset? The only challenge I can see in WoW is a challenge to ones patience.
On the fast levelling MMOs the challenge lies in doing the elite stuff (talking of both pve and pvp here), which requires skill over character level. And yes, PvE on those games is a challenge.
I can consider myself a pretty seasoned Guild Wars player, I have 9 level 20 characters (which is the max level there), and when I had time to play games, I leveled my characters in about 30\ hours... 3-4 days. And those were the boring ones: doing the missions I already knew, hearing the same story again, etc.
Being able to level them fast allowed me to try out the various play styles there is in the game, learn them to the point of perfection (I havent reached that point, but its the goal), try various team builds, various tactics etc etc. All this in PvE.
And guess what, I play that game for 2 and a half years now, half of those I played about 10 hours a day. Im a good player skill wise, and there are still areas that me and my regular group cant complete... and it isnt because of our level, but because of our play skill. 2 and half years later, with fast levelling, and I havent ran out of things to do in the game.
That is the real challenge those games have to offer, and they dont need to hide their lack of content (and I mean worthy content) behind a slow character progression.