I have a 6th Gen Nano and am running Windows 7, and have been dealing with these exact same issues for roughly 2 weeks now. Throughout all of this, I've regularly been plugging in my ipod classic, as well as a shuffle, with no issues whatsoever, so it seems to be some glitchy issue related specifically to the nano (possibly the iphone as well based on other threads I've read, so maybe it's a touch issue) and Windows 7/8. During this insane quest, I even tested it on a Mac, and it worked just fine.
Initially the Nano, when plugged in, would make the sound, pull up itunes, be visible under My Computer, but wouldn't show up in itunes.
A list of the things I've done: uninstalled every trace of itunes/reinstalled (numerous times), run a registry cleaner, played with the firewall settings, played with the startup items, uninstalled/reinstalled drivers, reset the ipod, restored the ipod, reformatted the ipod, tried different cables, tried different usb ports, tried everything possible in Device Management, googled every error message I've gotten....and even made 3 separate Genius Bar appointments at the Apple Store, all to no avail.
Once, I actually got it to appear in itunes, and celebrated like a mental patient, but then my elation quickly crashed when itunes wouldn't let me drag/drop anything onto it, then after ejecting went back to not recognizing it.
So...here's what I did that finally got this thing working. In an attempt to leave no stone unturned, I finally decided to try an itunes alternative, just to see what would happen. A quick google search led me to a Lifehacker article detailing the top 10 free itunes alternatives, so I downloaded the #1 choice, MediaMonkey, and after installing and importing my library I plugged the Nano in, and it popped up immediately on the left. I tested it by dragging/dropping songs onto it, ejecting it, plugging it back in, etc etc, and it works perfectly and functions more or less the same as itunes, so there's no real learning curve. I'm now thoroughly convinced this whole thing is just a glitchy itunes/windows compatibility issue that'll hopefully be resolved by the next version of itunes.
I hope this helps someone avoid all the frustration I dealt with.