My take on this:
Arrow straight roads (at least away from the mountains, which covers millions of acres here).
The DOT fell in love with stop lights, this results in traffic being a "batch flow" rather than a "continuous flow". People also accelerate at no more than 15% throttle up to speed, meaning you can't exploit any "fun" factor a manual trans might have.
90 degree intersections and the "tree of death" mentioned above being erected to prevent forward progress of traffic. Traffic circles and roundabouts used in Europe allow more potential for continuous flow. To say Americans are confused by these is an understatement. They put in a new one a year or two ago, I could hear the horns honking. Once people get used to them though, the improvement in traffic flow is dramatic.
Traffic on expressways and "freeways" often crawls at a pace where you have to slip the clutch, or leave a gap between you and the person in front. In this part of Michigan, if you leave a spot large enough for a whole car in front of you, a jerk (or several) will take it.
The "old fossil stoplight creep", where someone comes up to a regulated intersection and slows without stopping. This results in multiple clutch engagements for a single regulated intersection, in order not to leave a gaping chasm in front of you (that elicits horn honks here). Go to Phoenix this time of year for continuous demonstrations.
But the WORST BY FAR:
General lack of interest in the driving process. Given the above facts, most people have NO IDEA the performance abilities of their car. The "driving" instruction here should also be called "How to Obey the Traffic Laws" because that is what is taught. I fault many of my fellow Americans for their general lack of skill, but it's probably a combination of an absence of need and lack of education. Driving here is something you "have to do" in order to do the things you want to do when you get there.
Cars are an appliance. The most popular colors for cars here are white, black and silver. JUST LIKE YOUR FRIDGE.
Everyone has Facebook updates to get in, tweets to send, lunch to eat, baskets to weave rather than driving their car. This was enabled by many of the above factors, and seems to get worse with every generation.
Some more ranting:
The first mass production motorcycle (not a scooter or a super-scooter) I saw with an automatic transmission (the Honda VFR 1200) really made me sad. Until now, someone who rode a motorcycle had to learn how to run a clutch and change gears. Now that you can skip that, any brain donor can buy and run a bike.
I drove my Subaru STI to a party at my sisters house, and went to the store to get a replacement for a dish that went wrong with my dad, and left my keys because I was last in line in the driveway. It took us longer to do the errand than expected, and I got frantic phone calls from my sister. Of the 11 people there, NO ONE could drive a manual. Disappointed!