As a child I didn't really think about it so much but I do now find it a harder to be attached to very alien species that are quite different to us. For instance, if you were playing as space bugs then it's hard to really care when they are killed, hard to understand their motivation, hard to really get into it. Do you stick to the more human options?
I think as kids we only think about how cool it is to be a big dragon guy and how big their claws are but when we get older we want to have some understanding and connection with our avatar so we return to monk so to speak.
In the Halo 2 and 3 multiplayer, I always prefered playing as an Elite over a Spartan. Today I mostly just make my characters women, but when given the choice, I'll sometimes choose to be some weird alien or monster.