The Zeffirelli adaptation changes the words of the script somewhat. It is largely faithful, but not completely faithful. However, it does a great job being faithful to the spirit of the script. In other words, it is exceptionally romantic and poetic in sections like the balcony scenes, or when Romeo first sees Juliet, and slower and tragic in the death scenes. It is mostly faithful.
There is one huge change in the staging, and it does change the feel of the movie. It's the sex scene. The script never mentions them being naked, and there is no tradition of nudity at any point in this play. However, in Zeffirelli's adaption, you see a fairly large and shocking amount of skin. This is a major departure from the past, and changes the feel of the original (too much).