When writing for a largely American audience, I think it's fair to say that if I leave out any major descriptors of a person's appearance, most of my readers will assume that the person I'm describing is caucasian (meaning white, rather than, say, from the Caucasus). In writing contemporary fiction, it's easy enough to "tag" a person up front so that readers know not to default to "white." For example:
"A Pakistani gentleman walked across the room, clutching his briefcase in his hands."
Now, even absent additional description, the reader can orient themself in general on the person's appearance.
In writing a work for fantasy that is not meant to be grounded in reality, however, I do not want to put the following terrible, terrible kludge down on paper:
"Prince Tormai, who looked rather like a Pakistani, surveyed the room from his throne."
Kind of breaks the mood.
In that case, I need to default to a physical description. Dark hair, fair skin, blue eyes, green eyes, prominent cheekbones, freckles, and so forth. This brings me, however, to the following question, which fortunately isn't a problem for me during my current writing, but will come up in the next thing I work on:
What's a good, concise way to describe someone with East Asian (that is, covering the general range of Korean, Chinese, or Japanese) features accurately, such that people get the right idea, but without kludging it -- and without any easy outs such as having the person in question have an "Asian-esque" name?
In other words, if Linda Jones of the steppe tribes actually looks Mongolian, what's the optimal way to convey that?