Staying true to yourself, looking beyond appearances, and the power of friendship are big themes. (But there's also a fair bit of bodily function humor.)
Characters in peril; ogre hunters wave pitchforks and torches; a bird explodes; scary fire-breathing dragon (who is much less scary when she gets a crush on Donkey); one character is eaten in one gulp by the dragon.
Mild sexual humor. Some innuendo that will go over kids' head (for example, when Shrek sees the big tower that is Farquaad's castle and says to Donkey, "Gee, think he's compensating for something?").
Strong language for a kids' movie, including "damn," "ass," and "crap."