It didn't mean much to me at the time, but I started thinking about it while contemplating the possibility of starting a Hordeside Hunter. A 1-10 Hunter without a pet is a fairly unpleasant (if mercifully quick) experience, but that one is pretty obvious while others seem less so. There's a strange alchemy of level, quest drops, scaling, gear, dungeons, and skills that seem to combine to make life tougher in certain level ranges.
A 2005 guide to classes' relative leveling speed insisted that classes alternately sped and slowed as they aged and that the difference could be tracked statistically, and while I'm not sure I trust all of their math (and the information's outdated anyway), you can't fault the amount of work that went into it. Moreover, as the commenters point out, someone who picks a Rogue as a main is not necessarily playing the game with the same goals in mind as someone who picked, say, a Priest. A less scientific, but more detailed, look at leveling speed and class difficulties was written by Breanni of WarcraftPets.com, and her experiences seem to gel a bit more with conventional wisdom, particularly with respect to the speed of Hunter and Warlock leveling. That being said, Druid 1-20 was pretty ugly, Shaman late-30's is a parade of mana inefficiency, and I'm not looking forward to getting another Hunter to 10.