Basically, the QuickTime player has no more window. There are no borders, no permanent controls, not even a titlebar. When QuickTime is running in a non-maximised state, all you see is the content of the video, without borders, titlebars, and controls. When you mouse-over, the UI widgets appear, similar to the current full-screen interface. The titlebar will also appear on mouse-over - it's a transparent overlay.
In other words, Apple is kicking titlebars to the curb. The Safari 4 beta already did away with the titlebar, or, at least, combined the titlebar with the tab bar, which received lots of criticism by even the staunchest of Apple supporters, such as John Gruber. Removing the window frame and titlebar from QuickTime will probably lead to more raised eyebrows.