Some games are better than others at immersing you in the game world, and Kevin Carter has a piece up at Gamasutra detailing those little things game designers can do to make everything feel more real. And it adds up to the little things: "begin the level design process with your Non Player Characters (NPCs) in mind" (creatures shape and are shaped by their environments), "congregate creatures in groups around resources" (environments must be able to sustain their creatures) and "arrange like creatures in a hierarchical manner" (yes, you can have too much variety and it's not the spice of life).

So, what is the end result of the three guidelines provided above? Ultimately, if you tie creatures visually to their environment, congregate them around resources, and imply that they co-exist within a hierarchical relationship with other creatures, you are weaving the disparate elements of your game world together into one believable whole, which will ultimately mimic the real world—a mess of different elements connected via ecology.

Thinking back on the games that I've really submerged myself in, I'm not sure if they adhere to these principles or not - but like a lot of things in life, you don't notice when it's going right, you notice when it's going wrong. Maggie Greene

Living Worlds: The Ecology of Game Design [Gamasutra]