So I opened my "Mad Men" folder, which has TV shows from iTunes, and they're mostly 1.5GB files.
1.5GB for a broadcast show is >5Mbps.
So I went to look at U.S. residential broadband, and found this great article:
U.S. residential broadband speeds average 50% of advertised speeds
Using this chart, and some more math, it appears that 75% of U.S. broadband connections are not capable of streaming a 5Mbps HD stream.
Without downloading first, of course.