When looking at decisions a player makes in the game, there are two types: cosmetic decisions and impact decisions. Impact decisions actually have a numerical effect on your character, while cosmetic decisions don't really change anything.
It's interesting to see what a game chooses as a cosmetic or impact decision. An example of a cosmetic decision is character gender. I've played games where your gender changed your attributes (usually along the lines of males getting a bonus to strength, and females getting a bonus to intellect, or sometimes stamina). Yet in WoW, gender is a purely cosmetic decision.
In fact, almost every decision you make at the very start of the game is cosmetic. The only truely impact decision you make is your choice of class. Even race is mostly cosmetic. Sure each race has slightly different starting attributes and small abilities, but in the great scheme of things, race is mostly a cosmetic choice.
Indeed, the few places where race shifts from a cosmetic choice to an impact choice are the points of big debates and much forum angst. The classic examples are Dwarf priests (Fear Ward) and Tauren tanks (+5% health). Here's a point where an expected cosmetic decision actually turned out to be an impact decision, and that causes players to become unhappy.
Making almost all initial choices cosmetic has the big advantage that it's really easy to jump right in and start playing. You don't need to worry about every little thing.
Yet, I find that for a lot of RPG game designers, there is a tendency to want to model everything. If your character is fat, that should have a game effect. If your character used to be sailor, that should have a game effect. I actually like Blizzard's decision to make most decisions cosmetic, and only have a few significant impact decisions.
Another difference between other MMOs and WoW is character appearance when wearing armor. Other games often have things like dyes, which allow you to change the look of your character. This makes your character's look a cosmetic decision, for the most part. In WoW, by contrast, the look of your character is an impact decision. You usually chose armor by stats, and if your character looks like a clown, so be it.
This decision is an interesting one. I'm not entirely sure if it is a good one. On one hand, how you look is kind of the epitome of a cosmetic decision. On the other hand, it means that your appearance is constantly changing as you change gear. And for a MMO which you play for a long time, variety in the character model which is on your screen 99% of the time is a good thing, and can help retain interest.
It also provides a talking point for people. You can't really admire someone's hat, or commiserate with them about its looks, if you can easily change things. If you read the forums, when Blizzard releases images of new armour, especially Tier sets, there's huge buzz and discussion. Would we really have that discussion and interest if we could change the look of our armour?
Not to mention that there's actually a PvP purpose to it. Fixed armour looks provide a signal to your opponent about the quality or type of gear, and a skilled team knows how to take advantage of that.
So making a decision a cosmetic or an impact one is not always obvious. Generally though, if you set an expection that a decision is one type, people will be unhappy if it turns out to be the other type.
 A common question on the forums is "Human or Dwarf paladin?" The humans will point out superior racials such as Diplomacy. The dwarves will point out that the humans are pansies who can't hold their liquor. Both sides will walk away thinking they've won the argument.