I have done the obvious -- the USB driver was installed from the latest Android SDK, and USB debugging was turned on in the tablet.
When the Nexus 7 is connecte
I had similar problems and I found that unchecking all of the options (both MTP and PTP) allowed the device to get the RSA Fingerprint from my computer and after that point "adb devices" worked.
Keep in mind, the RSA fingerprint is required to be accepted before an Android 4.2+ device can connect via ADB, this is obviously for security reasons.