Birdemic starts with some terrible acting: Sure, that's amusing for about five minutes. But then comes the scene where the protagonist invents a solar panel that would single-handedly solve the world's energy problems, and everyone around him reacts like this is no big deal. Soon afterward, mutant birds appear that are made from those animated gifs they had on websites back in 1995, and then the birds start exploding and making cute little dive bombing noises while they do it. And it just keeps going. Amazingly, Birdemic pulls off this badness roller coaster for its entire runtime.
Of course, the boundaries of this rule are blurry, because not everyone is entertained by the same thing. Some people think Batman & Robin and Die Hard 4.0 successfully climb the "so bad it's good" mountain; for others, they're just bad. I enjoyed the Twilight movies on this level; a lot of people enjoyed them about as much as sticky floor popcorn that a hobo's been sleeping on. But it's important to realize that good-bad movies are like those trips to the dentist where they offer you a bunch of different of flavors of fluoride gel that all taste like butt: They should be varied in their terribleness. Another important rule is ...
They Should Give Us Insight into the Creator's Insanity
If truly good-bad movies aren't produced by the Asylum or the like, who makes the damn things? They're almost always produced by people who thought they were making good movies.
No matter what good-bad movie you're talking about, there was at least one person on set who thought that the movie is going to be great. In Troll 2, it was the Italian husband-and-wife director-writer team, who wrote their script in English despite hardly speaking the language. In The Wicker Man, it was Nicolas Cage, who poured everything he had into that scene with the bees. In The Room, it was director Tommy Wiseau, who seemed to think the story he was telling was a life-changing masterpiece.
"Whether 'tis nobler in the mind to suffer the slings and arrows of outrageous fortune, or to take arms against- oh ... hi, Mark!"