I did it: I ordered a new MacBook Pro.
So now I need a bunch of USB-C dongles and cables. Turns out that there's quite a bit to consider when shopping for USB-C cables. Here are some notes on what I learned. Please note that I couldn't confirm all of it, double check yourself as needed.
In principle, everything that applies to powering/charging USB devices over a USB type A port also applies to a USB type C port. So charging devices that take up to 12 Watts should be the same as before.
However, USB-C also has a Power Delivery specification. With USB-C PD, a charger can deliver 5, 9, 12, 15 or 20 Volts at 3 Ampères with regular cables and even 5 A with special cables, for a total of 60 or 100 W. All of this will be negotiated by the devices at both ends of the USB-C cable, and even the cable itself has a say because it's supposed to have a chip that identifies its capabilities.
However, be careful when you're using an adapter or cable to connect a USB-C device to a non-USB-C charger or computer. There have been reports of damage from this because the USB-C device drew more power than the other side could provide. So if you want to mix and match USB types, research your cables before buying.
The USB-C port has separate pins for USB 2 (480 Mbps and slower) and USB 3 and alternate modes such as DisplayPort, HDMI and Thunderbolt. So it's possible to have USB-C cables that only support USB 2 speeds as well as full featured cables that connect all the pins. As this requires 10 extra wires, this makes the cables thicker and stiffer, and very likely more expensive.
USB 3 originally supported 5 Gbps, but this was later upgraded to 10 Gbps. Thunderbolt 3 runs over the USB-C port at 40 Gbps for 0.5 m and shorter cables, 20 Gbps for longer cables. I tried to find out whether the higher speeds require different cables, but I couldn't find any information on this. So I'm going to assume that a quality 0.5 m cable will handle 10 Gbps USB and 40 Gbps Thunderbolt, and a longer cable 10 Gbps USB and 20 Gbps Thunderbolt. (Only expensive active cables support Thunderbolt over longer distances than 0.5 m.) So you probably don't have to pay extra for a cable that specifically says it supports 10 Gbps USB 3.1 or 40/20 Gbps Thunderbolt 3.
If you're buying a cable to connect to a charger, it's probably easier/cheaper to get a charger cable, especially if it's a longer one, such as 2m. I believe those still support USB 2 data speeds, but you'll want to check to make sure. If you want to charge a 15" MacBook Pro at maximum speed or run really demanding stuff on it, you'll want to use a 85 - 100 W charger and a 5 A cable.
15" MacBook Pros have always come with 85 W chargers but in my experience they run just fine on a 60 W charger. With a 60 W USB-C charger you should be able to use any USB-C cable, so that's much more convenient for a second charger.
I'm very interested to see how the new MacBook Pros handle smaller chargers. I ordered one that can deliver 30 W, which should be enough to run the 15" MBP off of most of the time, but it probably won't charge very fast. Or maybe the computer won't charge or even run off of such a small charger at all.
For data cables, I wouldn't pay extra for 10 Gbps or Thunderbolt-compatible ones. Very likely, cables that don't advertise these capabilities still support them. And you probably don't need those speeds right away anyway, so it's probably best to wait until cable compatibility gets clearer and prices are likely to come down in the interim anyway.