Here's the short list on how to measure the effectiveness of any marketing slogan or tag line:
1. Implies the main benefit of the product, solution, or brand for the potential user or buyer.
2. Implies a distinction between it and other products (legally).
3. Is short, traditionally, 5-7 words, although more recently 3-5.
4. Makes a simple, direct, concise, crisp, and apt statement.
5. Is often witty and adopts a distinct "personality" of its own.
6. Gives a credible impression (visual) of a branding message which is supported by the product.
7. Makes the consumer feel "good" as well as feel a desire or need.
8. Sounds good and is not easy to forget – sticks in mind.
Some of my recent favorites include:
Monster.com - Your Calling is Calling
HadtoSay.com - Because Biting Your Tongue Hurts
X-Files – The Truth is Out There
Finally, for a trip down memory lane, here's a list of classic tag lines I found at TagLineGuru.