Exactly. As far as I can tell, you cannot connect an external monitor to a USB port and have a GPU drive it. Thunderbolt serves a different purpose than USB: it is an interface between a small number of external devices and a small number of hardware components. The fact is that it can only at this point be useful for three things: displays, drives, and Ethernet. USB is what you use when you do not have another option. It is nice since it can support almost any kind of device because the protocol is not burnt into hardware, but of course the downside is that it has to implement the host side of things in software. Anyone trying to use a "USB video card" to play an HD video will notice how much of a toll it takes on the CPU. And when the CPU is fast enough not to be bothered by a 1080p video, we will instead be talking about 3D 8k video.
Basically, it is nice that USB just got faster, but IMHO direct access to the specific hardware (disk controller, video card, etc.) will always win.
Agreed. I used a USB 2.0 video adapter on my Air and while it was fine for a terminal, it probably managed 25FPS at best (Youtube is choppy). It also has to be by itself on its own USB controller (well KB & Mouse are fine) due to the Half duplex nature of USB 2.0. Using it on the same bus as a HD makes the display lag badly. Still cool that it can be done.
The new USB 3.0 ones probably fix all/most these problems but I just use monitors 1 & 2 for that ;).
Basically, it is nice that USB just got faster, but IMHO direct access to the specific hardware (disk controller, video card, etc.) will always win.