The Windows Vista Start menu abandoned the flyout model for the "All Programs" menu because nested fly-out menus are a usability nightmare, and not just for novices.
Research has shown that once you have menus more than one level deep, you have the problem that the slightly wiggle of the mouse can take the big, complicated menu hierarchy that the user spent enormous attention to build and make it all disappear in a flash. I run into this a lot. "File, Open Multiple, By Searching..." oops I moved my mouse too far upwards and tickled the View menu and boom my menu vanishes and I have to start all over again. Menu navigation has turned into one of those mouse dexterity games where you have to guide your character through a maze without hitting any of the walls or you die and have to start over.
The All Programs menu has turned into a unwieldy mess thanks to all the programs that shove themselves into every nook and cranny. As a result, navigating it as a hierarchical menu has turned into a common source of frustration due to the "collapsing menu" problem. The solution? Reframe it as a tree view which acts only when you click. The hierarchy is still there, but it's much easier to navigate.
This ease of use comes at a cost: If you're one of those people who can guide a mouse with pixel-perfect precision, then you're going to find mouse-based menu navigation a bit slower due to the extra clicking. But if it's speed you're after, then put down the mouse and stick to the keyboard: Type the name of what you want into the Start menu search box, and you'll be taken straight to it.
(No nitpicker's corner today. We'll see what happens.)