I pick my race based on RP concept. Examples:
Sh'lieulias is an elven necromancer. I wanted him to be very weak, but very smart. I also wanted him to be a bit unusual. Humans just aren't weak enough to me; elves on the other hand are very guant, but rather fluid and very intelligent. Thus, he was an elf.
Vence was human as he was a rough from the Mikona slums. It was a natural choice, though IC he's about the size of a half-orc.
Keint I made sort-of as an experiment; unfortunately, I didn't do a very good job of RPing his gruffness. I'm thinking about taking up with him again and making him a harder, gruffer, more veteranish character.
Samantha Cobble is human because I wanted her to be fairly normal and average. She has no score below 10 and originally had no score above 16. I also made her human because I have a hard enough time RPing a female, let alone a non-human one.
Two common traps I see are people playing too much to their race. People sometimes feel too constrained by their racial boundries. I also have noticed that lots of people have human, elven, and dwarven characters; humans and elves (and the crossbreeds) I think are very common because people feel the least restrained by them, whereas dwarves tend to be a bit more stereotypical in general but are fun to play. If you look though, at least as far as I see, the vast majority of characters are humans and elves, with humans having a bit of an edge over elves. Gnomes are the rarest character; perhaps it is because they are a weird race that people aren't sure how to play, or perhaps it is their weird racial adjustments (+2 con, -2 str); all I know is that I have seen about a dozen gnomes, and only a handful of them more than once. I have been considering creating a gnome character just to counteract this, but I must admit that I run into the same roadblock with them as everyone else in terms of creating viable, long-standing concepts.
In custom races, I have noticed that the most prevalent custom race on Avlis seems to be kobold. There aren't THAT many of them, but there are a fair number. Perhaps it was Mr. Kutah who inspired so many others to play them. There have been a fair number of ogres (or are they half-ogres?) as well. There are quite a few ghost elves (possibly more than kobolds), but no one can tell them apart. Moreover, Sh'lieulias is at times mistaken for a ghost elf, to the point that I'm leaning towards him having some ghost elf blood even if he is not one himself. I have seen one wemic (myself, though I have heard of others), only heard of centaurs, met one lizardman (Sidac), have yet to meet an orc, have seen one gnoll... CCs aren't very common, perhaps because it is so hard to come up with an independent RP concept for them.
On Abyss 404, there are lots of custom race characters, but a lot of them play once and quit. There are two bantmar who are around a lot, and two or three tieflings. I think that Abyss 404 lends itself more to those, though, especially given that they are native/more related to the outer planes than normal humans/demihumans. I'm sure there will be a higher proportion of CCs there than anywhere else, possibly because there is very little RP background known for some of the races so people seem less constrained in becoming them, and also because it has had CCs since its rebirth so they are permanently embedded in the psyche of the world.
It is harder to RP non-humans in my opinion, and I think that is part of why there are so many humans. I think it is a good thing though - humans are the most versatile race, and we get all sorts of great characters from their ranks. Also, they breed like rabbits, so its no wonder there are so many of them.
