On Nook HD charging cable - Charging over microUSB has some limitations - that includes a USB standard max amperage (I think around 1.5a up / .5amp down).
The Nexus 7 is actually one of the few tablets I've seen that charges on standard MicroUSB. Most tabs have something proprietary. Galaxy Tabs and the like have a proprietary charger. Nook Color had microUSB with a couple of extra pins (to help it charge faster...it takes about 8 hours with a standard .5amp usb cable and the screen has to be off to get more power than it's using).
http://www.extremetech.com/computing...our-smartphone
Covers it a bit.
To charge a battery the size of the one in the Nook HD (6000 mah I think) would take a very long time. The Nexus 7 only has a ~4000 mah battery IIRC. I assume that they wanted HDMI out which is why they didn't go with the Nook Color/HD style cable.
faster). iPads use their proprietary connector. Etc.