The USB able to supply power, and although it only supports 15W by default it can optionally go up to 100W, which means you can, say, connect your notebook to a Thunderbolt 3 monitor and have the monitor also charge your notebook through the same connector, something you can’t do with Thunderbolt 1 and 2.
The Thunderbolt features have also been enhanced and you now get an absurd 40Gbps of bandwidth, twice that of Thunderbolt 2, along with 10GigE Ethernet connection.
Thunderbolt 3 also features 2x DisplayPort 1.2 lanes, which allows a single Thunderbolt 3 port to carry two DisplayPort 1.2 signals, meaning you can connect a 5K60 MST display or two 4K60 SST displays on a single Thunderbolt port.
