Laura Ries tells us:
The mission of a slogan is not just to define your brand, but more importantly to differentiate it from other brands.Read Laura's entire post. It's a great lesson on positioning with many concrete examples.
One way to test the differentiation factor is to reverse the slogan and pin it on the brand’s major competitor. Does it make sense? Could it define another brand? If not, then the slogan is just plain puffery that is likely to be ignored.