The Dock-to-Lightning transition was painful for iOS devices, because many of us had invested in an ecosystem of devices that relied on the Dock connector. Most of us swapped our iOS device when we got a new iPhone, iPad, or iPod touch (remember those?), and in a home social grouping, would pass down an old model. Lightning meant our stereos and cables and docks wouldn't serve old and new, yet the old devices still had plenty of life in them.
Worse, most people I know with an iPhone have two or more cables, sometimes permanently installed in different places, like a car, or stashed as an extra in a satchel or purse. The early Lightning adapters were $29 for a little stub and $39 for a cable. You could wind up with incompatible audio and other gear, and $150 of cable costs. It annoyed people, and rightly so, because it felt like an upgrade penalty instead of benefit.
But the situation is different with the MacBook, as you won't lose a lot of sunk costs if you're shifting from one Mac laptop to the MacBook. The only interface types you lose are MagSafe and Thunderbolt. If you need Thunderbolt devices, this isn't the computer for you. Your MagSafe adapters, meanwhile, can clearly continue to be used with your older computer, whether you keep using it or pass it on.
USB-C allows bidirectional charging, which changes the cable equation. The new computer ships with just a USB-C charging cable (two meters, $29 sold separately) and a 29-watt power adapter with a USB Type A jack ($49 separately). The charger can power an iPhone or iPad, and ostensibly the USB-C charging cable could be driven by any USB Type A jack--although a 5-watt or 10-watt charger or a 10-watt or 12-watt car adapter will charge it quite slowly. (I was unable to get confirmation on that latter point.)
With a USB-C adapter that splits into multiple interface types, you can charge devices over its USB parts just as if they were part of the computer's hardware. Apple is offering three adapters to start with: a $19 USB-C to USB 3.1 Type A port, into which a regular Type A connector can fit; and $79 A/V adapters, one for VGA and one for HDMI, both of which also sport USB-C charging and USB Type A ports. (It's unclear at this writing if that USB-C charging port can be chained into more adapters, but it seems very likely based on the spec.)
Apple isn't offering gigabit ethernet or DisplayPort adapters in its initial foray, but these adapters should be available soon from third parties. Last year, such adapters were expected in early 2015, and this availability projection may be one reason Apple isn't shipping the new MacBook until April. Its spec sheet says that the MacBook comes with "native DisplayPort 1.2 output," which will support 4K (3840x2160 pixels in dual monitor or mirroring modes), but there's technically no way today to access that stream of video data.
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