(1) Candor (and/or) personal vulnerability (and/or) humor in your message
(2) Relevance of imagery to the text. How well are the words integrated into the image? Does the image offer additional layers of meaning to the overall message through contrast, irony or storytelling?
(3) Surprise & something unexpected
(4) Creativity. Can you think of a twist in how to say or communicate your answer?
THREE things I've learned in soliciting the most compelling answers from people.
1) DIG DEEPER WITH EACH PERSON. Ask them to tell you verbally their answer before they begin writing a sign. Listen for specific wording and interesting phrases in their verbal responses and then suggest that wording right back to them. I've found that people at first often come up with a general answer that can sound clichéd (e.g. "For love"). But, something happens if I just ask a few more whys... "why did you say that?" and again, another step deeper, why did you think of that reason for saying "for love"? (and so on... with more whys) People may stumble a little when getting push back like this, but they are usually really thankful in the end for the dialog. And, eventually everyone comes up with something much more idiosyncratic that reflects THEIR PERSONALITY MORE.
One good example: someone said "to change the world" and after asking a few more "whys" and having a brief conversation, he told me about how he was kicked out of seminary b/c he was impatient with the slow progress of change. So, at the end of our conversation, he came up with "Too rebellious for priesthood," which I think is fantastic. And, we had a great dialog, during which I learned much more about him.
2) ENCOURAGE VULNERABILITY. People often want to answer wdydwyd? in a positive, aspirational way. That is cool. I like that people want to put a stake in the ground for something positive b/c they may live into that hope with more vigor. BUT, I also think there are many other motivations that aren't so positive (we're all pretty complicated). The value is that people can identify the fact that they are not alone in feeling what they're feeling.
I've tried to spark those sorts of answers as much as possible, and sometimes it takes encouragement to get people to do it. There have been a good number of anonymous entries submitted from around the world, and they are usually quite moving. Here is a page I created specifically for such a challenge: http://wdydwyd.com/anonymous But, people don't always need to do it anonymously to be vulnerable.
3) TELL PEOPLE THE MOST COMMON ANSWERS. A simple tactic is to tell people what the most common answers are. Most people don't want to be just like everyone else. It also improves their answers.
These include: "why not?" "because i can," "because I love it," "it makes me happy," "for my family/children," "for love." All of these are good sentiments, but inevitably, people can add a twist that makes it more reflective of their personality.
Please contribute (below) any feedback on these suggestions or any of your own tips that may help others. I'll add them to the tips above and we can have a living, growing document (It's a collaboration!).