UI

I feel UI has the biggest impact on immersion than any other element.  I hate bad a UI and I love a good UI.  So what makes a UI bad or good?  First lets talk about bad UIs.

Bad UI clutters the screen with ugly windows.  They distract from the game play rather than enhance and enrich it.  Bad UI requires the player to come out of the game experience in order to check inventory, viewing maps, or changing spells/abilities.

A good UI tries to maintain the realism and immersion of the game.  It tries to integrate the concepts of inventory, maps and abilities into the experience.  Tom Clancys: The Division is a game that is doing a fantastic good job of this.  All the UI elements you see make sense, they add to the game rather than distract.