Crap movies, as I explained try to be good. They are usually mainstream movies trying to appeal to all audiences. Most of these movies try to be good. They try to distinguish themselves as cinematic masterpieces and fail. Usually their downfall is cookie cutter plots, stilted characters, wooden acting, too much dispense of disbelief required, etc. Mainy of the mainstream horror movies comming out in this day and age fall into this chategory. Movies like The Cave, Boogeyman, Dark Water, The Grudge, and The Ring all have serious flaws in them that separate themselves from the great horror movies of our time. Movies like The Texas Chainsaw Massacre (not the horrible remake), Dawn of the Dead (actually either remake or original is good), Evil Dead, and 28 Days Later. Most of these movies have some of the following flaws:
Too much use of children. Notice that most of the horror movies that come out today all have something to do with children. Either the child is the one who senses the evil presence, or the child is the monster. Let me say this right now, "Children are not scary. Children have never been scary. Children never will be scary." Using little kids to scare audiences just seems stupid. I've never seen a 7 year old kid and wanted to run away. Adults and demons are way scarier.
Too much reliance on the demonic being. Usually the thing that is scaring the people in the movie isn't all that scary. Most people are scared of the thought of the being rather than the being itself. In fact, sometimes no visible monster is the way to go. In Evil Dead you knew there was this evil presence in the forrest that was possesing the people of the cabin, but you never actually saw it. Possibly the scariest thing in the world in not knowing who or what your enemy is. Remember the movie Signs? You never really saw the aliens until the movie was almost over. That was a good technique. It made the movie scarier when you never had a clear picture of the invaders. The alien itself was nothing. Play to that. Only give your audiences obscure pictures of the being.
Leave the audience wondering. A lot of mainstream movies like to wrap up the movie with either everyone being dead or the monster being dead. They like to show you that it's all over and done with. At least until they decided to make a sequel no one wants. A few things I like: When the people don't kill the monster, just escape from it. Remember the final scene from The Texas Chainsaw Massacre when leatherface is swinging his chainsaw around as that one chick finally escapes? That was a good scene. Leatherface is still out there. The story may be over for the girl, but it's not over for anyone else who goes out there. Or Dawn of the Dead. The planet was overrun with zombies. The characters managed to get out of the mall, but they were off to unknown territories. Who knows what their fate will be (this is why I recommend you don't watch the credits in the remake as it answers that quesion for you. That's the remake's only downfall).
It's not just horror movies that are bad. Ever seen Dude Where's My Car? It tried too hard to play on the complete stupidity of it. It wanted to be stupid and it was. But it relied solely on the stupidity of it to succeed. So it ended up just being crappy.
Now trash flicks are entirely different. Trash flicks are usually only bad because they have a low budget. These are the indie films of the world. These films, unlike mainstream crap, don't try to hide how bad they are under big names or special effects. These movies rely only on themselves as a whole. Movies like The Toxic Avenger, Bad Taste, Army of Darkness, Tromeo and Juliet, and Ichi the Killer fall into this chategory. These movies are good because they don't care if you like them or not. These movies aren't the safe, MPAA aproved, politically correct features that plague today's cinema. If one of these movies wants to say something, it will say it whether or not you like it. These movies don't try to shield you from anything. Filmmakers here don't compromise their vision just so audiences will like their movies. They make the movie they want to make. And that's why trash flicks are always worth watching. The reason people usually call these movies "bad" is because they are either turned off by the low budget quality of the flick, or because they object to something presented in the movie. I like to call these people #$%@'s.
So stop putting crap flicks and trash flicks in the same category. They do not belong together.