So, in my video editing class today I gave a lesson I've taught before: we take the famous "Odessa Steps" sequence from Sergei Eisenstein's Battleship Potemkin -- 7 minutes and 19 seconds long -- and work to cut 30 seconds out of it without ruining it.
It's a great process, and for those of you who enjoy editing I suggest you give it a shot. (You can grab the Eisenstein film over at archive.org.)
In any case, every student has their own ideas of what shots are crucial to the edit, and which can go away. Sometimes people are very conservative -- just making shots shorter, but keeping them -- and sometimes they go right for elements you or I might say are critical to the story ... and cut them right out.
It never fails, however: at least one student misunderstands: "Here it is," they proudly say, "I cut it down to 30 seconds."