1)Thumb placement-thumb should not wrap around fret board. Thumb should be placed on the middle to the lower third of the back of the neck. This is very important in the early stages of learning in that it makes the far reaching chords easier to attain. As you mature and become more comfortable and fluid with your chord changes the occasional thumb reach around will be acceptable and useful.
2)String control-be aware of strings ringing out that aren't intended to do so. Example; many students of mine when playing a C chord play the low(fat) E as well. This, in most cases does not work and it sounds sloppy. Muting or dampening strings (not in the framework for a particular chord )with the palm of your strumming hand is something to be aware of.
3)Not looking at strumming hand- Look at the fretboard and develop a feel for strumming or picking. Also, take some time away from the guitar and visualize chord placement in your mind. It really helps.
4)Guitar jargon(language) If one goes from the first fret to fifth fret, one is moving UP the fretboard. If one is going from the fifth fret to first fret, one is moving Down the fretboard. Many people confuse the direction and it drives me crazy(as an instructor) It goes by pitch, as the sound gets higher, Up we go. As the sound gets lower, Down we go.
Good luck and don't forget the most important rule of playing guitar; love and cherish every moment a guitar is in your hands!!!!!
Give everything but up.