I'm of the the 'anything goes as long it's within the law' school when it comes to cinema. There seems to be few truly adult (as in definitely not meant for children, not the blue sense) films anymore. Now and again I want to see films that haven't been made with an eye on the lucrative teenage market and are as gory as they're able to get away with. The BBFC is probably the strictest - and I mean that as a compliment in this context - film censorship organisation in the world. They simply don't allow cruelty for cruelty sakes scenes in UK films. If, in their considered opinion, a scene is in there purely for that purpose, they will ask for it to be cut. If the film-makers refuse, it's likely not to be granted a cinema release. Films like I Spit On Your Grave, Cannibal Holocaust and the like would never get a cinema release today (not that they did in the 70's either) because they would fall foul of this.
Anyway, I digress. The point I'm making is that most people know what they're going in to see and what to expect from watching the trailer and reading a review. I can't believe anyone would see Saw II 'cold' thinking it's a film about DIY. True, it may turn out to be more intense than you expected, but you can always leave. Personally I avoid anything by Richard Curtis because I know I'm going to hate it.
Also, films don't always have to be entertaining to be very good. Schindler's List is in no way entertainment, nor is something like Capturing The Friedmans, but they're undoubtly very good films.