Another common problem: The Android Developer Site does not tell you to change the device storage settings from "Media Device (MTP)"   to "Camera (PTP)". This is necessary for ADB to detect certain devices (Nexus 7.)  I know we send them to the Android site and this seems like a deficiency in the Android docs, but I don't think it would hurt to include a tip like:

TIP: After following the instructions to set up your Android device for development, it is possible that ADB will still not detect the device. Ensure that your device's storage settings are set to "Camera (PTP)". On Android 4.4, this can be done by going to Settings -> Storage -> (Click the drop down menu on the top right) --> USB Computer Connection --> Camera (PTP).