S550: It's the Little Things - HUD
Heads-up display (aka HUD) was originally created for fighter pilots. Now, this display projected onto the windscreen is a standard luxury auto feature.
The HUD display in the S550 is easily turned on and off using a button on the panel to the left of the steering wheel. I generally leave it off for in-city driving (where I find it distracting) and turn it on for highway driving (where I find it useful). I'm particularly impressed by how much information Mercedes has crammed into a very small space, thanks to clever display design. Your current speed is always the biggest number, but it also shows your cruise control (Disctronic+) speed setting, using both a number and an orange marker on a virtual speedometer gauge.
The HUD is particularly impressive when you are using the built-in turn-by-turn navigation system (which, while not as good as Google Maps, is still surprisingly good for a 7-year-old system). Mercedes manages to show you your next turn, how far it is (including a diminishing countdown bar when it's close), and also tell you what time you're expected to arrive. All cleanly presented, in a way that doesn't unnecessarily obscure the road ahead.
The only down-side is that windshield replacements are expensive, as you have to use the OEM Mercedes windshield that has the proper treatment to reflect the HUD display.