So I got a new 925 - see another post - and I plugged the USB/micro plug into a front USB port on my PC. Started charging, no surprise. After awhile, I noticed it wasn't. The phone charges fine from either the micro USB data type connection or the wall charger.
Apparently is was some kind of safety built into the circuit, it works fine again today.
Here's something indicating this thread is on the right track. My Battery Sense app shows that when last plugged in, it was the wall charger, it started at 48%, was at 100% in 59 minutes! This is a 2000mah battery, ergo, charged 1000mah (1 amp/hour) in one hour. The wall charger is rated at 5V, 1.3A out. Since charging involves losses, i.e., you waste some electricity due to inefficiencies, I'd say this all makes sense.
I remain skeptical that a micro USB is good for having 2 amps pushed through it, but I guess Nokia is certainly OK with 1.3.