Watch the The vi-IV online guitar lesson by Ravi from Guitar Lab: 30 Rock & Pop Progressions
One way to alter the I-IV chord and add interest is to substitute the I chord with its relative minor, the vi. We can take the vi-IV and put it in any key, as we can do with any progression. By doing so, we often find easier fingering positions and also move into a key that might better suit a particular singer's vocal range. Plus, the minor quality gives a bit of a darker quality, adding a new dimension.
Using barre chords instead of open positions can make transposing keys easier, as well as give us a different sound. By adding some more interest in the strumming, we can really give an identity to a commonly used chord progression and capture the feel of the song.
Examples: Eleanor Rigby, Listen to the Music, Rhiannon