I liked it better when they did not auto play. It allowed me to open tabs in the background for each video and article I wanted to view at the beginning of my visit, and work my way through them after that.
I have currently reactivated FlashBlock on this site for that reason.
One reason I can guess for the change is the same reason youtube may also do this. Page hits are what they most likely use to calculate their traffic volume. It is important to them that they can see what stories, and videos, have the most traffic.
If people open 12 tabs and then read through them later, as I do, then there is a greater chance the person got distracted and ended up not finishing their reading/watching. So if they see 300,000 views of The Big Picture, it's possible that say 10,000 opened the tab and then forgot about it and didn't view it. Maybe it looked good at the time, but then the moment passed and the person then didn't care about it anymore.
That's all a guess though.