Imagine that you're playing a game with incredible graphics, sound and a killer plot, but you're not having fun at all...in fact, you're getting pretty bored. The answer lies in the small details that make a game fun to play. Sure, it might have oceans, but where are all the fish? The jungles are lush and lifelike, but where are all the sounds of nature, or for that matter, any sign of nature at all? The enemies have killer AI, but they seem like robots, never talking, or yelling. A game can have great grapics, gameplay, and sound, but if you don't go the extra mile, the player will never feel quite like it's much more than a game. Imagine this: you're walking down the street, but you don't hear anyone talking, there aren't any birds in the sky, and there are no cars driving down the road. Pretty boring and empty isn't it?
You don't, however, have to go berzerk when adding detail; a simple fish swimming in the water adds more realism than you'd think, and a leaf blowing in the wind makes a scene very real and lifelike. Think about it, does life ever really come to a complete lull? NO! There's always something going on, be it a bird chirping, a car going down the road, or a bug walking across the ground.
Here's a list of different things you could put in your game that aren't very big or hard to draw, yet will have a dramatic effect in the realism in your game: a fish in the water, a frog on a lilypad, a leaf blowing in the wind, a fly buzzing in the air, a sparkle on the ocean, (or anything that reflects light for that matter) a cloud passing by, etc. On a closing note, you don't have to have wonderful graphics (though those are a plus) to make an immersive game, just a little extra effort and a lot of imagination.