There is a fine line that separates a movie from being so bad that you can't watch it and being so bad that it's a joy to watch. Movies that often fall into the latter category are many early attempts at science fiction movies or more recently, disaster movies. From the penultimate classic, Plan Nine From Outer Space, to the fourth or fifth in the Airport series where Karen Black's stewardess character has to land the plane, they have a certain je ne sais quois about them that elevates them beyond awful. (What the makers of Airplane, the supposed spoof of the Airport franchise, failed to realize was the impossibility of looking more ridiculous then the original movies.)
Perhaps it is the cheesy special effects that make them so great. Not only are the flying saucers made out of aluminum pie plates, but you can also see the "invisible" fishing wire that's suspending them against the painted backdrop that is obviously moving. Or how about the great acting where all the women sound like they're on respirators they're so breathy in their delivery, and all the men talk in earnest monotones?
The scripts aren't in any danger of winning Pulitzer Prizes, of course, with the plots being simplistic and filled with scintillating dialogue like, "Damn aliens" or the ever popular "We come in peace". Of course what really makes these films are the little things, like a complete absence of continuity and camera operators who seem to be suffering from such severe delirium tremens the movies seem to have been shot during an earthquake.
While the majority of the movies that are so bad we love them were meant to be taken seriously, occasionally a modern filmmaker will see fit to pay homage to the genre and try to make a movie that captures all the things about them that we loved. Unfortunately, modern equipment is almost too good and it's hard to make a movie complete with jerky shots and unsynchronized dialogue.
In order to make one of these paeans to B-movies successfully, filmmakers have to have genuine affection for the genre and the ability to do something on the cheap. But most important is to make sure the movie is shot in all seriousness and let the idiocy of the script and the scenario speak for itself. If it is played for laughs then it no longer has the same appeal and becomes merely another silly comedy.