In the description of some races there are creature origins. For example gnomes have the Fey Origin and devas have the Immortal Origin. Are these just for flavor or do they have a real mechanical effect? Has anyone come across any rules that have to do with creature origin? Are these rules too rare to worry about?
How often creature origin actually comes up in your games depends entirely on your players/play style. But here are some things that are affected by creature type.
* Monster knowledge. If a PC wants to make a knowledge check to learn more about the monster, the type of skill used depends on the creature type (natural creatures are nature, and so on for arcana, nature, religion, dungeoneering).
* Some rituals, particularly binding rituals like magic circle can be keyed to certain creature types.
* some powers, the easiest example is turn undead, do specifically target a specific creature type. most of these really are divine class powers (anti-undead being the most common), though I think i've seen a druid utility power that does something with fey creatures.
Of course, if you all don't use rituals, and no one picks the powers that are creature type specific, and your group's playstyle doesn't roll for monster knowledge checks very often, then, really, it is possible to go for several campaigns without it ever being an issue.