I think this is annoying during movies I love and take very seriously. I'd be annoyed if anyone laughed at an inappropriate time during, say, Titanic, or Moulin Rouge. But I can be a bit of a hypocrite if I don't personally love the film myself
I was giggling a lot in some of the emotional parts of Harry Potter and the Deathly Hallows: Part 2 (the return to the Gryffindor common room! Neville's fucking horrible speech). Now that may have pissed a lot of the die-hard fans off, but it was under-my-breath laughing, not LMFAOing in the middle of a packed theatre.
People were pissing themselves in my sitting of Paranormal Activity 3. It damn near killed the movie too. That's probably the worst movie audience I've ever witnessed.