Ths Simpson's movie has a lot of big shoes and expectations to fill.
For one, it is the longest running TV series (comedy) in the history of TV as well as the longest running animated series in TV history as well.
There were rumors the original concept of the movie was going to be live-action like "The Flintstones" and other cartoon adaptations... But this rumor proved false, ultimately (they did do a nod to it though in this LIVE ACTION sequence, however).
On a personal note, I have faith in the writers because while the recent episodes haven't been that good, this usually means they are keeping the best material for the film as writers often brainstorm more than one idea per epsidoe which we don't get to see normally and will probably end up in the feature film.
I also hope the film gets a PG-13 because then they can just go "off" on everything and everybody -- Like the "Family Guy" movie ("The Untold Story of Stewie Griffin") did when it was released direct-to-DVD and was rated R because of the irreverant and controversial topics they parodied like race, religion, George Bush, Iraq, etc.