MASH is one of Robert Altman's widely acclaimed films and I am one of the many people who do not get the greatness of this movie. MASH is defined as hilarious, ground-breaking, audacious, surprising, anti-war, etc. Well, I can agree it has been audacious at the time of its release. As for the rest of it, I am not quite sure. MASH has its funny moments but it is far from a film I would call great. I consider it a vastly overrated movie.
If you look into IMDb's description of MASH, you will see the movie is about a field hospital (Mobile Army Surgical Hospital) whose staff uses humor and sex to escape the terror of the Korean war. MASH teems with disobedience, misogyny and disputable jokes. How we are supposed to know though that keeping their sanity in a good state is the doctors' actual motivation behind their behavior is beyond my perception of the movie. Indeed, there are lots of jokes, many of them really funny but I do not see how they make this an anti-war film or a film about people looking for salvation from the horror of war. The setting for these jokes could have been pretty much anything else and it would have (or have not) worked nearly as well.
The lovers of MASH have a variety of arguments in their bag: the movie has been very efficient at the time of its release but many things have changed in the decades that have passed; the film is funny and horrifying simultaneously and the often brutal jokes are result of the doctors' hopelessness; the same persons would behave differently in a "sane" world.
Now, these arguments are not completely devoid of sense but they are not very strong either. For example, there are many older movies about war but their age doesn't make them less anti-war or worse. Stanley Kubrick's Paths of Glory is a good example for an older and much better movie. And if we have to rely on the funny moments in MASH in order to appreciate it, I have to disagree once more that this is a very successful presentation of the terror of war. Just take the Academy Award winner for Best Foreign Language Film from 2001 No Man's Land and you'll have a movie that is a way better in every aspect - it's both funnier and more horrifying than MASH while at the same time it demonstrates the actual drama and insanity of wars.
Of course, MASH has a few virtues. Some of the jokes and the situations are comical despite often showing cruelty or indifference. The cast is not bad at all featuring Donald Sutherland, Elliott Gould, Tom Skerritt, Robert Duvall and Sally Kellerman who has been nominated for Oscar for her role of nurse "Hot Lips". Most of these actors have been in far better movies though, playing better roles and demonstrating better acting. Finally, airing audacity and anti-establishment is the movie's last quality I can think of but it is considerably devalued by time.
So, if you are a fan of Robert Altman or you want to see all of the films on the 100 Years...100 Movies list of the American Film Institute, you have a reason to see MASH. If you want to see a good comedy, drama or anti-war film there are better options. The aforementioned No Man's Land is just one of them.