'Filthy' is a slang term that has both positive and negative connotations. In the case of the top 3, 'filthy' means 'hot, clean, sexy, awesome, killer.' Which are all positive. These posters are successful because they communicate the message of the film, they're well-executed compositionally, and they're eye-catching.
There's a massive trend in today's film poster design--photography/imagery directly lifted from the film. Not too many graphic executions anymore, which is why posters like the ones below KICK some serious BUTT.
Rocketeer. No words can describe this complete and utter kick in the gonads. It makes me wanna be a better person. Very, very nice.
Pulp Fiction. The poster harkens back to the days of old pulp magazines, complete with wrinkles, wear and tear, and price tag. Love how the credits at the bottom are skewed. The vintagey feel could be handled more effectively, as the 'N' in 'FICTION' appears to lay on top of the distressed wrinkle, but all in all, this poster encompasses the film in one well-handled execution.
And then there's these. In terms of message, these definitely communicate. Comedies tend to be less about design and more about just sheer ridiculousness. 'Encino Man' tells us that the dude is a caveman. We know we're going to watch a movie about a caveman in today's society. I think the poster is absolutely perfect. Perfectly dumb and simple, just like the movie. They didn't overthink anything. I don't like over-thinking anything, either. If the answer is too hard to figure out, it's probably the wrong answer. Things should just MAKE SENSE.
And then there's 'Caddyshack.' Just check out the yellow. It can't get any brighter. And the photoshop job of the dudes coming out of the ground...not bad! This is in the early 80's after all. Again, no high-concept here. Comedies don't need it. Nobody wants to see an abstract gopher shape coupled with a golf hole and shack in the background. There's no jazz hands here. and for that, I'm grateful.
Not sure why the ghosted gopher in the BG is so massive, though. He is larger than life in the film, always snatching their balls and dancing to tunes, so maybe that's the designer's way of giving an ominous presence to the little guy.