Say what?

The 5 Gb/s is marketing speak, the real, raw data rate is 4 Gb/s. The PHY has 200 ns symbol time (5 Gsymbols/s) but encodes the data with 8b/10b, i.e., every 10 symbols on the wire corresponds to 8 bits of information.

If you rated 802.11a/g the same way it would be 72 Mb/s. The 54 Mb/s data rate uses a 3/4 coding, i.e., for every 3 bits input, you get 4 coded bits on the air. (4 us symbol time, 288 coded bits per symbol, 216 data bits per symbol.)

This sort of exaggeration seems to be becoming more common. I sometimes hear that HDMI 1.0 is 4.5 Gb/s, when it is really 3.6 Gb/s (TMDS encodes 8 bits for every 10 symbols as well).

The good news is that the 4 Gb/s is bi-directional, there are separate up and down link wires, as opposed to USB 1.0 and 2.0 in which the same wire and hence the available bandwidth is shared for up and down link.

The protocol for USB 2.0 is not particularly efficient either, which is part of what gives the much lower throughput for a single device. The aggregate throughput (i.e., with more than one device) can be higher. OTOH, the simplicity of the protocol makes endpoints cheap.

USB 3.0 looks to be a much more efficient protocol, so it is possible that the throughput improvement will be significant, aside from the improvement in the PHY rate.