I think cryptic crossword players would be particularly interested in this. There is a significant amount of kudos associated with solving an unusual clue and therefore perhaps more desire for users to share solutions with the community/collaborate on solutions. I know avid fans of the Times (UK) crossword who pass their part-complete solutions back and forth throughout the day, your app would be an easier way for them to collaborate. For more details see http://en.wikipedia.org/wiki/Cryptic_crosswords