*** Language ***
Some reviews mention 'mild swearing' and some mention 'constant swearing' so how does a parent know who is right? Here are some statistics pulled directly from the subtitles: sh** is said 4 times, d*** 16 times, bas**** 5 times (why can the main review type that word but I cannot?), a** 3 times including used once with the word hole, hell is said 8 times, b**** is used 3 times, God is used as an exclamation 12 times including twice followed by the word d***, Jesus is used as an exclamation 9 times including twice followed by Christ. These were just the words it occurred to me to search, this is 58 different instances (not counting compound swears twice) of profanity/taking the Lord's name in vain - even if the latter doesn't bother you (not everyone is Christian), it is still 39 times. In addition to this, other reviews mention numerous examples of name calling (like peckerwood).
*** Violence/sexual content ***
Sorry to lump these together, but in the case of this movie, there is overlap. Doc is killed/appears to be killed by terrorists with automatic rifles. Marty's dad is a peeping-tom in a tree with binoculars (watching Marty's mother in a state of partial dress). Marty suggests he pretend to date-rape his own mother so that his father can be a hero and rescue her. Biff attempts to rape Marty's mother. None of these were particularly graphic.
*** Positive messages ***
Marty is a great kid who (generally) tries to do the right thing.
*** Final thoughts ***
An edited version of this movie could be great for kids and adults alike. In my opinion, the language it is too pervasive even for me to want to watch it with my wife without being edited (at 112 minutes long, it averages something I do not want to hear every 1min 53sec [not including name-calling] - worse than many PG13 and even many R rated movies). If people taking the Lord's name in vain is particularly offensive to you (as it is to many Christians), you will likely be quite offended by the pervasiveness of that in this movie - at 19 times it averages once every 5min 53sec. I also found it interesting that Common Sense Media says the movie has content age-appropriate for 8 year olds, but yet there were 31 usages of words that they do not allow in their reviews.
I do not claim this review to be comprehensive, an apologize if I have offended anyone's sensibilities - but please note, the movie is more graphic than this review of it (if the review offends you, do not watch the film!).