Most of my characters have been human - though I'm proud that my completely racially mundane human sorcerer was one of the strangest and most disturbing members of the party in the Planescape game which was my introduction to Third Edition, because he became an alienist. Beating out a half-fiend githyanki who became a lich, a werefox, a minotaur, a half-fiend, a half-celestial, a githzerai dragonfighter and a rogue modron psion in the weirdness stakes was no mean feat.
I tend to play human characters for a few reasons. First, I've rarely been inspired to play characters whose concept required being from a nonhuman race - in the example above, my alienist wouldn't have been more interesting if he had been an elf or a gnome, so I wouldn't have seen the point of taking a nonhuman race even if I hadn't had the motivation to be different by playing a normal human.
Second, most of the nonhuman races available in games I've played have been pretty dull. I'm not jazzed about core D&D's treatment of elves, dwarves, halflings, et cetera; I think the Eberron treatment gives them an interesting twist that I could get into, but no-one's running Eberron in my gaming circle (and I'm the most likely to do so).
Third, often I just find myself playing in games where humans are the only option - such as the d20 Wheel of Time campaign I played in. Similarly, Hong's "Britannia 3E" game had some nonhuman races, but they tended to be very close to humans - the main option being one of several varieties of spirit folk - which kind of makes the point of playing one disappear.
That said, I have played my share of nonhuman PCs. In a brief Forgotten Realms campaign designed around the idea that everyone in the party would have at least +3 Level Adjustments, I played a grey orc with the blooded template; in an even briefer and very canon-light Spelljammer campaign I played what amounted to an Oriental Adventures Rokugan-style naga; I'd love to play a warforged if anyone ever runs an Eberron campaign I can play in.