Given that this movie was put together in less than a year might explain its shortness (81 minutes - including end credits, so roughly 76 minutes of actual film). But what it cannot explain is its lack of humor that the previous film possessed.
The gags are quick and sometimes not even funny. The only true funny parts are the quick spoofs on the Nike basketball spots, James Woods' portrayal of Max Van Sydow's character in the Exorcist, and bits and pieces scattered throughout the film. Very unfunny was the take off of Charlie's Angels, which like the first Scary Movie and the Matrix spin off scene, basically recreated the scene without much humor injected into it.
Today's youth might not be able to relate to the spoof gags of the classic supernatural horror films of the 70's such as the Exorcist and maybe of the 80s' Poltergeist, et. al.
Hopefully Scary Movie 3 will take some time to put together, making the spoofs more enjoyable.
One thing though, the film features more than the last one of promising young actress Anna Faris (whom I will admit seemed exceptionally hot in the sequel). Just for her casting and acting ability, I give this movie a "3" out of "10".