The way I see it, there's a difference between willing suspension of disbelief, and complete disregard for believability. The context matters here, too. Films set in the "real world" (e.g. cop movies) will have a more difficult time taking liberties with things like physics and other issues of believability. Films set in the far future or a fantasy setting can always appeal to "Well, it's not this world," or "Well they have magic/high tech stuff." For example, I don't really care that you can hear the ships fly as they dogfight in Star Wars, nor do I care that the pilots are basically dressed as if they were flying in an airplane, rather than in sealed suits suitable for ejection into the vacuum of space. Whatever, it's ok, it's entertaining, and maybe they've got some fantasy-sci-fi thing to protect them.
But some stuff just starts to get stupid, even in such a film. Different people have different thresholds, but for me, there's a point where the action sequence transitions from "exciting" to "ludicrous." Typically, this happens when the goal is purely to create some exciting sequence, without any attempt to ground it or have it make sense or provide context for it. Case in point: the jaegers punching the kaiju in Pacific Rim, and only forming a sword LATER in the fight. They never provide a context for it or explain why they're fighting that way. They don't say that their tactics and designs were oriented around less durable kaiju, or that the sword is a one-use emergency weapon, or whatever. They just form the sword after almost getting blown up, and while it's exciting on the one hand, it leaves you saying "Wait, WTF?! Why didn't they do that earlier?!"
Other stuff just makes the movie flat-out stupid. Many of the CGI/wire-enhanced stunts in Live Free or Die Hard were just....stupid. He hit a woman with a car, who then got up and fought him in a fistfight. FALSE. YOU ARE DYING OF INTERNAL BLEEDING IF YOU ARE NOT KILLED OUTRIGHT BY THE INITIAL IMPACT. Some other guy jumped from a helicopter hovering 30' up and landed without a scratch. FALSE. YOU HAVE SHATTERED BONES AND TORN LIGAMENTS IN YOUR LEGS. Those are my reactions when I see that kind of stuff. It goes well beyond suspension of disbelief. I can accept that a tough, NYC cop is able to continue fighting even though, over the course of the film, he's strained himself beyond the breaking point of any actual human being. But you gotta dole that out to me over the course of the film. But when Maggie Q gets hit by a car and survives? No, sorry, that breaks reality.
See, films strike an unspoken bargain with us when we watch them. They promise to entertain us, but they promise to do so by telling a story that is at least operating on an internally consistent basis. The world must, within its in-film context, be believable. Which means you can't tell me one set of rules and then change them mid-film. Like, you can't tell me "This is the real world" and then FLAGRANTLY violate the laws of physics. I mean, if the hero gets shot in the head, but shrugs it off and keeps going, you'd better damn well explain why and how. Are his bones coated with adamantium and he has a healing factor? Is he made of liquid metal? Is this a video-game simulation world that he's learned to hack? You have to provide context.
Too many films ignore the contextual rule-breaking and assume "It's just a movie" is enough to get by on the fact that a 7-round pistol somehow has 49 rounds in it, and that the hero can still lift his arm and aim perfectly even though he got shot in the deltoid.
