[Thunderbolt Q&A, Help & Troubleshooting] mike.s:
Close, but not quite.

The pins are +5, Ground, +D and -D. All power is drawn through the +5 and Ground lines. The D lines are used for data transfer.

Per the USB specifications, a device can only draw 100 mA without enumerating (negotiating via USB, implies both ends have "intelligence"). If it can enumerate, it can negotiate for up to 500 mA (e.g. plugged into a PC which has a "driver" which recognizes the phone).

There are also specifications for DCPs (Dedicated Charging Ports). For these, the D+ and D- pins (the center two in a full sized USB connector) on the charger must be connected together with no more than 200 Ohms. These ports must provide at least 500 mA, but can provide more (micro-USB connectors are rated to 1.8 A). That's how the phone knows it can draw more than 100 mA from a "dumb" power adapter.

The TB follows the USB charging spec. Here are some things that can happen.

If it's plugged into a charger which doesn't follow the USB spec (D+ and D- not connected together, these are fairly common), it can only draw 100 mA. It doesn't matter if you bought a "2.5 Amp USB charger," if the charger doesn't follow the spec, a device which does isn't supposed to draw more than 100 mA. That's enough to charge it very slowly when the screen is off and it's idling. If the phone is doing anything, the battery will actually be discharging. This is a problem which seems to be common with many car chargers. A lot of device will ignore the 100 mA spec limit, and draw whatever they can, which is why they will charge from an improper adapter.

It's plugged into a PC with no driver - same thing.

It's plugged into a PC with a driver - it can negotiate and pull 500 mA. This is enough to charge while running in almost all cases (it might be on the edge if streaming video via 4G with GPS on, etc.) It will charge, but not as fast as it could.

For all of the above, a TB will show "USB" as the charging type.

It's plugged into a USB DCP. By spec, these must be able to deliver at least 500 mA. The AC adapter shipped with the TB is marked 1.0 A, and the phone won't draw more than 1.0 A (the most I've seen it use to charge the battery is ~800 mA). This will allows the fastest charging. With a DCP, the TB will show "AC" as the charging type.

Cables can have the same effect - all USB cables are supposed to be basically the same. But, some manufacturer's play loose with the spec. They may have "USB" cables with resistors or other changes inside which signal different things to a device. They may not have proper gauge wiring. It may just be a bad cable. Etc.

I've charged the TB with the stock AC charger using both the stock cable, and one I got from Monoprice. Works fine. I've also got a USB hub I converted to a charging station (follows the spec for USB dedicated charging ports), and that works fine with both the original and the Monoprice cables.

I have run into "flaky" cables, where they only work right if jiggled around a bit. Maybe they've been flexed too much, and have an intermittent connection. Maybe the contacts have gotten dirty. I don't worry about it, I just throw those away, you can get new ones from Monoprice for less than a buck.