I'm thrilled that Microsoft is opening up game development to non-professionals. I believe that the destruction of a steady path from enthusiast to power-user to programmer was one of the great tragedies of the 1990s. But the idea that XNA will be "YouTube of video games," conflates "point videocamera at self and lip-sync," and "write videogame."
The hobbyist game development language, to the extent that the niche exists, is Flash. Microsoft's XAML/WPF is a heck of a run at Flash, but as far as I know, it's not going to be a part of XNA. So hobbyist developers will use, presumably, the gamut of .NET managed languages: C#, VB.NET, C++/CLI, and, I imagine, will be able to bring in at least assemblies written in other languages like IronPython, Delphi, and so forth.
This is my guess: XNA will appeal to people with a talent and interest in programming, who will quickly discover that gameplay design is an entirely different discipline. They'll become frustrated with that idea and search for an arena in which their programming talents shine without requiring similar investments in gameplay. The result: the return of the demo scene.
Demo programming is programming (generally graphics intensive) that highlights the capabilities of the underlying hardware and the talents of the programmer. Traditionally, demos have involved constraining the program to a fixed size, but XNA for the XBox 360 has the even-better advantage of evening the hardware environment. Since all XBoxes have equivalent computational and graphics power, programming graphics on them is just like "one-design" racing.