I've seen quite a few Shakespeare productions, in person and on film, that seems to have cut 40% or more of the text and that I still thought were good to great. The Zeffirelli film is a horrible cut of the play, IMHO. The only cuts of any Shakespeare play that I've seen anywhere and thought were comparably myopic and disrespectful to the text were The Hollow Crown, which is also hugely popular. Zeffirelli's Hamlet sucks too, and some of the same problems, but it's better than his R&J. His Taming of the Shrew was my first experience of that play and I liked it at the time, but now I'm afraid to rewatch it.
Also, that R&J is pretty long and boring. Yeah, most Shakespeare productions/movies are that long or longer, but I've seen a student production of R&J that I'm pretty sure used more of the text, and certainly used more of the story, and was at least 40 minutes shorter. It was also immeasurably more enjoyable.
For dubious editing decisions on a Shakespeare film that I still like, I'd point to the 1960 Tempest that stars Maurice Evans, Roddy McDowell (who would soon star together in Planet of the Apes), and Richard Burton. It moves the "Our revels now have ended" speech until the end, and cuts out the actual closing monologue! I guess every existing copy of this movie looks as shitty as it does here, so don't worry about trying to find a better one:
Speaking of Shakespeare productions that make hash of the play, have you heard about this?
It sounds even worse than the RSC version with the swinging metal ball. It'll probably be available to steam at some point.
Shakespeare’s challenging text has been streamlined to a neat two hours in this contemporary production.
I think the play is more challenging when it presents a moral landscape that is messy and ambiguous.