Certifying Wi-Fi Direct as a standard ensures interoperability, meaning devices stamped with said certification can use their own wireless connectivity to connect directly with each other, without requiring access to wireless networks. In other words, a Wi-Fi Direct certified laptop, for example, would be able to send documents to your printer (just one of the two connected devices needs to be Wi-Fi Direct certified) and keep your phone's contacts up to date (most products certified will support "one-to-many" connections), without having to talk to your router.
Range will eventually be a major selling point; it's reasonable to expect that future Wi-Fi Direct devices will be able to achieve distances similar to our home wireless networks. It's also important to note that while 802.11a/g/n is supported over 2.4GHz and 5GHz bands, there's no requirement for Wi-Fi Direct products to support 802.11b.
Bluetooth will undoubtedly be the first technology to suffer as a result of Wi-Fi Direct, though we should see fewer USB connections around as well. Since Wi-Fi Direct will be able to use the same transponders as other Wi-Fi functions, device manufacturers will be happy to remove redundant technologies in order to cut costs.