[For a definition of "diegetic," see this post]

I'm impressed by Fallout 3's interface.  It's the most diegetic-focused RPG that I've ever played.  The three-dimensional real-time nature of the engine makes it very immersive, but what really gives the game an advantage over titles like the Elder Scrolls games is the pip-boy.  The wrist-mounted pipboy serves as your character sheet, inventory, map, quest list, and probably more.  Aside from time pausing when you bring it up (a necessary feature for control), interacting with it is diegetic.  It makes the game flow quite well.

The combat system also works well.  If Fallout 3 was a shooter, I'd hate it simply because of console controls.  But it's an RPG, and shows it with VATS, another semi-diegetic system that essentially lets you pause, queue up attacks, and then execute them.  It's almost turn based, and seems very true to the previous Fallout games.  No twitch here, thank goodness.  VATS' time-stopping nature does slightly damage flow and reduce immersion, but for a console game I wouldn't have it any other way.