that makes sense. I'm glad I get to know more of these stuff. I have a follow up questions and I hope this is the appropriate place to ask it. I guess I was paying attention to the wrong spec on the charger. Here are the spec sheet.

Asus wall charger(Nexus 7):   input 100-240~50/60Hz 0.3 A    output 5.0V 2.0A
Apple wall charger(iPad 3):     input 100-240~50/60Hz 0.45 A  output 5.1V 2.1A

now this is what I noticed. Of course the devices(iPad and N7) charged with no problem using their own chargers. Additionally, the iPad charges using the Asus charger but the N7 wont charge using the Apple charger. I can see that the output of the Apple is a little more (voltage and amperage) than the Asus. Can you explain this if you can? I hope I'm not asking for too much. Thank you in advance. 

I really didn't care about the rear facing camera when I bought it as I have never used one on my iPad. This device is a smaller form factor though and I find myself bringing this more often compared to my iPad. There are a few occasions that I wished I had a rear camera to share photos to family and friends of something I saw at the very moment.

I would love to have extra storage space. Either give us a 32GB or 64 GB internal memory or an SD expansion slot. Again, I came from an iPad and have been pampered with extra storage space. But I think the better of the 2 options is the expandable SD card slot so you can have extra space after you buy the device, this sounds cheaper to do in the long run as well.

last but not least is 3G/LTE connectivity. I think the reason I need this compared to my wifi only iPad is that again, I don't usually carry my iPad everywhere but I find myself carrying this often. Also this is the perfect device to use as a portable GPS navigation system and the best of the best software out there uses internet connectivity.

I will try and make this as clear as possible because it is hard even for me to see the pattern of how charging works for this device. So far, these are what i have:

cables:
OEM cable,
6-ft cable,
1-ft cable,

chargers:
OEM wall charger,
iPad wall charger,
portable battery charger(with both .5Ah and 1Ah sockets),
USB car charger.

so far these are the combination that works:

OEM wall charger + any of the cables I have available
any charger + 1ft cable.
Macbook air + any of the cables I have available

This means that my portable battery charger and car charger only works if I use the 1 ft cable.

I understand that chargers have diff amperage and that might be the reason why some works and some doesn't. I'm wondering what the diff in the cables that I have though. How come some of them work and some don't? And why is my 1ft cable even better than the OEM that my N7 came with? I've had Apple devices for the longest time and they are all interchangeable. I thought micro USB would be the same. I was mistaken. Can someone here shed some light on this matter?