Well, "Set Poster Frame" is still there. But it requires resaving the file every time. (I would think it's just a flag pointing to the frame number. Why must the whole file be reencoded?)
I'd settle for the damn preview icon choosing something more randomly in the middle of the video as opposed to one close to the beginning. So many of my videos look the same because they all START THE SAME. Choose a frame further in for the love of Mike! Like YouTube picks one in the middle. 50% through. OS X seems to pick one at 5% or even 2% into the video.