I've been partly surprised and partly not surprised by the number of people who've responded yes to this question. Many have said that they hadn't really thought about it before, but actually they do. A few have said that they definitely have, and feel slightly bad about doing so!
I always had a sneaking suspicion that people did, simply because I do it myself! Being an outgoing girl, I've met quite a few of my in-game friends in reality, and was not remotely surprised when my friend @ameurling turned out to be a tall, pretty, blonde girl. Of course she was! She looks just like her priest!
So, before I become frightfully boring and start theorising about why we do this, what do you think? Of course, I'm aware that you probably realise your friends aren't actually tauren or draenei or goblins or worgen, but do you picture them having at least some of the physical characteristics of those races? Or not at all?