Shrek is merciless in its humor. Targeting everything from fairy tales to Disney films to narrative clichés to bad puns, sliding in its own commentary, all the while giving the typical fantasy story a few modern twists and turns to deliver a strangely original unoriginal story with original unoriginal characters . . . that doesn't make sense, but Shrek does, does it well, and doesn't care. And you never know where this humor is going to come from, either either visual gag, musical nod, or spoken dialogue. All three provide their share of narrative and commentary to the concepts in Shrek.
I think the beauty of Shrek is it's taken the typical fairy tale (which all of us have heard), and it answers the silly 'what if . . .' and 'why don't they ever . . .' questions we tend to ask. But that's not the core of the story - the core of this story is the friendship between Shrek and Donkey, which works in a funny goofball, but touching and unique way. If the audience can buy into Shrek and Donkey's relationship to one another, then they can believe entire film (which more or less focuses on the adventure the two share together). True, Shrek has a love theme . . . but it's Donkey and Shrek who sustain the film through most of the picture.
As for Cameran Diaz and John Lithgow, while not on screen as much as our two heroes, still play an important role. I wouldn't want to downplay Lithgow and Diaz who do bring their respective characters to life, but Shrek is a tale about an enduring friendship with a romance story on the side.