7ths are not always positive for a song.
The 7th in G (which is the so-called dominant in the key of C major) can easily give a song a slightly "folk song" or ballad feel. Sure, if you're playing Cornelis Vreeswijk's "Turistens klagan" or something by Alf Prøysen, this is not a problem. In that case, the effect of the 7th is welcome and fits in. But if you're playing more towards pop, rock ballads, etc., then G7, in the key of C, does more harm than good.
In that case, try not to play a 7th chord at all (unless a part of the song necessarily requires it). At least not G7 as in our example. A G7 is consequently played as a pure G major chord in that case. This way, you get a slightly more modern sound for the song.
