This is the question to which anybody tries to find an answer. Some users reported that their player changed in a way, others that it changed in another way, so it's a very confusing situation. I found that it has been changed for everybody in a way, but some videos do actually have another, much cooler interface. The second one looks something like this below.
I didn't find anywhere an explanation for this, but I suppose it might be because the player is not yet adapted for some resolutions or qualities of the videos. Anyway, it gives a fresh look to YouTube, but I really don't know why they're still working on Flash, as the HTML 5 is coming strongly behind.