Interesting how you found the back of Millennium Force to be the worst seat. I am the complete opposite. Here is my theory as to why you felt that way. First off, I would be interested in knowing what time of day your other Millennium Force rides were. Millennium Force is almost useless to ride early in the morning as your only back seat ride was. It is so much faster at night! It speeds up as the day goes on. And if you rode it before the rest of the park opened, then I am assuming that you rode in a rather empty train. Heavy roller coaster trains travel faster than lighter ones due to air resistance. And since Millennium Force has such a high average speed and since its the longest steel roller coaster in the country, the difference is more noticable on Millennium Force than most other roller coasters. The bottom line is that Millennium Force is a faster more intense ride at night, or just even later on in the day, no matter where you sit.
Personally like El Toro to name another, I find Millennium Force almost a waste of time in the moring. I always save it for night.