What tiny thing in Lion makes you smile or has caught you off guard?
Using the FaceTime camera to add signatures to PDFs in Preview. 

Click the annotations button in the toolbar and use the drop down menu next to the signature icon to grab your signature from a piece of paper you have written it on. Then just click and drag in the document to place it. Haven't really needed it yet, but it's implemented so nicely that I did it just for fun.