Reading the comments I received on my recent post about playing against your real life gender, I noticed that many people made mention of the fact that males in many games tend toward ridiculous excesses of mesomorph formation -- unlikely musculature, epic height, etc. On the other hand, when they're not saddled with over-large breasts, females are much more sensibly structured with regard to proportion.
Even so, there aren't any overweight avatars. There aren't many options for height. Perfect skin, perfect posture, no deformities, etc. World of Warcraft plans on offering more variety in hairstyles with the upcoming expansion Wrath of the Lich King, but is that really substantial? Would you like to have more options in creating your avatar? EVE Online goes a little farther, but you never see your character after that anyway. City of Heroes/City of Villains does a great job of it -- is there a reason why they can't all do the same? Would you like Second Life-level customization in your MMO of choice, or are you happy with what you have?