One of the most effective techniques for making games immersive is also one of the most simple: Just add a clock.
Developers have a number of ways of pulling players into their game worlds - impressive graphics, realistic physics, relateable characters - but sometimes all it takes is making the sun rise and set. Instilling a sense of time into a game, argues Dan Squire in Issue 216 of The Escapist, can build a bridge to the real world in ways that few other elements can. An in-game time cycle can even add new layers of depth, simply by turning day into night:
Around this period, time's primary function of helping to create a dynamic, life-like world became more apparent. In-game time was primarily used to create a sense of adventure, grandeur, and freedom. Open-world games and games that emphasized exploration embraced the idea, while more story-intensive genres continued to partition day and night only when it made sense in the narrative.
Squire's certainly right that having a day/night cycle is a relatively easy way to add a surprising amount of depth into a game. Running around Cyrodiil at night is certainly a vastly different experience than exploring during the day. It just feels more quiet and dangerous. Read A Persistence of RAM and share your thoughts on the topic.