Keen and Graev have a good post up about why (according to them) players prefer playing DPS roles. Statistically, it appears to be more or less true-- according to Warcraft Realms, four of the five highest class percentages are traditionally DPS classes: Mage, Rogue, Warlock, and Hunter. Warriors also have a higher population, but it could be argued that only 1/3 of the Warrior specs (Prot, as opposed to Arms or Fury) out there are actually meant for anything other than DPS.
So why do players seemingly prefer to play DPS? K&G give three main reasons. They cite something they call "Big Number Syndrome," which is the idea that unless you're dealing big damage, your class is worthless. They say that doing DPS requires less responsibility-- tanks and healers have to pay attention to everything, but DPSers choose a target and kill it. And they say that DPS classes level faster, which seems anecdotally (at least) to be true-- more damage means a faster kill, which means XP more often.
In general (very generally, in fact), I tend to agree. For these reasons, some people are definitely drawn to the DPS lifestyle. But I don't think that these reasons are why people chose these classes in the first place. Hunters, for example, have pets, and I think that's a much bigger draw to the class than "big number syndrome" ever was. And let's not forget that these are more or less the most archetypal classes in the game-- someone who's never played the game probably would immediately know what a "Mage" or "Rogue" could do, whereas a Shaman (the lowest class population, according to the census) is a little harder to explain.
So I think K&G are putting the chicken before the egg-- these things may be true about DPS looking back (and they may in fact be reasons people choose DPSers as alts). But when people first choose a class to call their own, I think it's a little simpler than that.
[ via Hardcore Casual ]