Defining Greatness

Considering the subjectivity involved in what it is we do, I've often wondered to myself what makes a great ad? Recently, I spoke with a couple writers at Martin-Williams who stressed the need for every campaign in a student book to be great.

When they know you've spent a couple years devoting your life to building a portfolio, you shouldn't just be doing good stuff. But what if some say your work is good, but others say its great? What if some don't even think its good?

I say a great ad is an ad everyone (or most everyone in the great creative schema of things) agrees is great. For example, the above headline issued in the 2000 CA Ad Annual by Carmichael Lynch--no creative in their right mind could honestly say that this isn't a great ad, or at least a great headline.

Thoughts? How do you define what a great ad is?