I switched to 8.1 it's really good once you get rid of all the Metro crap and have a start menu. (Start8) First improvement I noticed is multichannel SMB. All my files are stored on a local server...I wouldn't go back to Win7 because of that!
This is because mobile hotspot and tethering use are not allowed with unlimited plans (except for Connected Cars). If you want to use your device as a mobile hotspot or for tethering, you'll need to change your plan or move the device to a plan that supports mobile hotspot and tethering.