Simply put, Thunderbolt's a familiar-looking port, a brand-new chip, and a cord, which allows devices to pipe two data streams simultaneously -- in both directions -- over a single cable at up to 10 gigabits per second to start, primarily using PCI Express x4 for data and DisplayPort for video.
The announcement of Thunderbolt instantly reminded me about the old Firewire vs. USB battle. If you have read the comments or even the press release about Thunderbolt, you have probably seen how people are comparing Thunderbolt to Firewire, and the battle is now between USB 3.0 and Thunderbolt. There are certainly a lot of parallels but I do not know if there is a clear winner, or even if there will be one that dominates the other.
Thunderbolt definitely has more impressive specifications but will that be enough? USB has a far larger install base and is likely to remain much cheaper to implement for various peripherals. Does my mouse really need a 10Gbps lane? Will a Thunderbolt based keyboard cost more than a USB one? I personally suspect that the two will live together, with USB remaining the dominant connection type, with Thunderbolt serving the more demanding uses like external video cards, displays and hard drives. What do you think? Is Thunderbolt the future? Or do we all live in a USB world?