Short answer: entertainment value.
You see, I've watched a lot of bad movies. Many of those movies were panned by critics, and I loved them. Many of them were panned by my friends. And I still loved them. And many of them were loved by critics and/or my friends, and I thought they were the most boring waste of my time in the history of film. Obviously, it's all subjective. But what I always want in a movie is to be entertained. And it really only has to have a 60% Entertainment Index for me to say, "Yeah, I liked that."
Now, I can hear you, sitting at your computer, reading this, saying, "Giles, we need an example. What does a 60% Entertainment Index LOOK like?"
Well, I'll tell you, lucky reader! For me, it's not enough to be entertaining some of the time. And, for that matter, if all 60% of that entertainment shows up in one solid chunk with the other 40% of the film just sucking, then there's a good chance I'm not going to like it. We're not looking at a bell curve or a rising graph. We want something more like a sine wave, but unbalanced, with the peaks higher than the drops. If it's as low as 60%.
But that's not really an EXAMPLE, is it? No, what I'm talking about is best described by Pacific Rim. I loved that movie. It's robots vs. monsters, for crying out loud! But taken from an "objective" analysis of it's individual parts, it should have been a colossal failure (and box-office-wise, it was). The writing was sub-par with several plot holes, mediocre dialogue, shallow character development, and contrived story events that didn't flow naturally. The acting left something to be desired, too. But it was beautiful! And what it lacked in writing and acting, it made up for in excitement, energy, and engagement. For me, it was entertaining and exciting. Even my complaints couldn't overcome how much I loved it.
One the other hand, we have movies like Revenge of the Sith. Which I only saw once because the writing, acting, and visual effects felt shoehorned and unnecessary for the story itself. Yes, there was an awesome lightsaber fight. One of the best in movie history! But aside from Ewan McGregor, the performances didn't engage me. They stood out as stiff, awkward, and forced.
So, what does a good movie need? Engagement. Entertainment. And a story that's good enough to make up for all of its other failings.