Comment by toast0

The problem with USB, is the revisions are all encompasing. So USB 2.0 includes the 1.5 Mbps and 12.0 Mbps modes from usb 1.x as well as the new 480 Mbps mode, so you could have a USB 2.0 device that only did 12 Mbps (high speed!). It might actually be your old usb 1.1 device with a new label.

PCI-e requires hosts and devices to be backwards compatible, but the interface speed is a required part of the spec. Nobody makes a PCI-E 2.0 device that only works with PCI-E 1.0 encodings/speeds, or anyway, it wouldn't be acceptable.