Tags should describe the content of the question. They should cover the specific problem area to be addressed. Having a problem with a particular app? Use the tag for that app. (Examples: google-plus, whatsapp-messenger.) Having a problem with a particular function? Use the appropriate tag(s). (Examples: gps, bluetooth-tethering, mobile-data.)
Tags should not describe your device. That is, tags for
- the model
- the manufacturer
- the version of the operating system
- the mobile carrier
should (probably) not be used.
While this information is important and should be included in the question post, most of the time the issue is not specific to the device or the OS version. Only when the issue is clearly tied to the environment should a tag of that nature be used. (For example, for quite some time AT&T disabled the ability to install apps from "unknown sources" and was the only carrier to do that. A question regarding how to get around this restriction would be correctly tagged with att.)
And, of course, avoid meta-tags.