In the USA we often call G major The People's Key. That's because it is well suited for the group singing of many basic I-IV-V folk songs such as Oh Mary Don't You Weep, It Takes a Worried Man, Got to Travel On, She'll Be Coming Round the Mountain, Jesse James, and This Land Is Your Land. The most important chords (including G, G7, C, C7, D, D7, Em, and A7) are all easy to play on the standard 6-string guitar, and do not require any barring. Picking out bass runs or the melody (or a skeleton of the melody) with the thumb or with a flat pick is usually fairly simple. The main chords are also easy to play on the 5-string banjo when it is tuned to an open G, and the melody can often be picked with the thumb and forefinger.