What's stopping the ability of charging a cell phone from another cell phone? If I were to create a crossover/patch USB cable and connect it between two phones or tablets, would there by any way to have one of their USB ports provide a charge to the other?
It's fully possible to do that if the charge-from device has USB On-The-Go (OTG) support.
You just need a USB OTG host cable (like this one), which you connect to the charge-from device, then plug a normal micro-USB cable into that, then into the device to be charged.
The only obstacle you may run into is current limitations. A USB OTG port almost certainly will not provide as much current as an all-up USB port, much less a charger.
Best case, this will mean that the charging device will charge slower.
Middle case, you won't get enough current to charge the device while it is powered on.
Worst case, the charge-from device will simply say "You're drawing too much current. You're cut off."