I'm not a professional film critic or anything, and I'm not trying to comment like one, but I've got something I'm pretty sure on why some movies are so great: They simply never stop on get better as the runtime go.
I saw the movie because of its reputation. I knew its something good and i wanted to find out why everyone can't stop praising it. So when Henry Fonda pulled out that switch blade and stuck it on the table, i thought "oh thats it, that's why they say this movie is great, a huge twist". And then came the old guy with great insights about the old man downstairs and the woman across the street, they were on 2 to 10; And then there was the man who shouts his hate for people from the slumps, everyone in the room showed him how ignorant he was, they were tied on 6/6; And finally, the last man teared up his photo with his son he haven't seen in 2 years, sobbingly say "not guilty" ... Everything built up like a pile of random acts at first, but as the movie progresses everything fell into place like a luxury box of Belgium chocolates. I'm not saying you need to be keep on getting better to be a great movie, a lot of classics don't work the same way (or even the opposite), but if a movie can build up like 12 Angry Men, you just can't be bad.