Many Faces of the QFN
When I first started using a CAD package for circuit design, I couldn't understand why the software didn't come with all of the standard footprints and why that wouldn't be good enough. That level of ignorance was hopefully pretty short-lived. But back then, things were a bit simpler. Most of what I dealt with were in DIP packages - 0.1" lead to lead spacing and 0.3" or 0.6" width.
Even if the reality was never that simple, it's a lot more complex now. Take the simple QFN. Your CAD package probably has a decent variety of QFN footprints, but most of them probably look like these first two here.
On the left is the generic footprint with a wide open flag pad in the middle. On the right, the cross-hatched area is likely representative of what the solder paste layer looks like for that standard library part. Looks simple, but there's a whole lot more to the QFN (and DFN).
Take these next two images. The image on the left here shows what should be a pretty common standard solder paste layer for a QFN center pad. You want to keep the paste coverage down in the 50 - 74% range to prevent the QFN from floating up during reflow.
Some QFNs, especially high-frequency and RF parts require a special copper pattern to ensure proper grounding and clean signal conditions. This one on the right shows the recommended copper pattern for a particular Freescale ZigBee radio chip.
Even if your CAD package seems to have the QFN covered, check the component data sheet for any special land pattern requirements and check the paste layer to make sure it has you covered to prevent float or voids.
One face, two face
Red face, blue face