Wait what? I really don't see how this one wasn't light-hearted, especially considering we got a fairly long, upbeat musical number right in the middle of it all. Considering just how preachy and annoying this episode could have been I think they did a very good job of keeping this one light and enjoyable.
Aside from the musical number there was some great stuff packed into this one: Peter matter-of-factly saying "I hate Quagmire lately", Peter and Carter's propaganda commercials (especially the Hitler one), the "Above the Influence" commercial parody, Lois getting a shot in at Brian after bailing him out of jail, the closure to the cat plot and definitely Peter's "these are all the people I hate" cutaway.
Good stuff.