Making sure the strings are the right height, all the way down the fretboard. Making sure each string is intonated correctly - that it plays in tune at every fr

Making sure the strings are the right height, all the way down the fretboard. Making sure each string is intonated correctly - that it plays in tune at every fret. (For mechanical reasons, guitars always have problems being in tune everywhere, even the best guitars. Have you noticed that the bridge is not straight, relative to the frets? That's because each string is a slightly different length, to help get it in tune better.) A good setup should also make sure the tuners are moving freely and accurately, that no more substantial repairs are needed (fretwork or major neck adjustments), etc. For a beginning player, these differences can be hard to discern, but they do help. For an advanced player, a poorly set up guitar is a pain in the butt to play.