For some reason, the old interface was more subtle, while the new one seems to be more in-your-face. Now that YouTube tests a white background for video pages, the new player stands out more.
Here's the experimental player:
... and the player from the standard interface:
To try the new player, change your YouTube cookies using the instructions from this post. There are some other changes: new options in the "upload" drop-down, "now playing" is replaced by "what to watch", YouTube now shows the number of subscribers and there's a new way to display the number of likes and dislikes.