I'd like to nominate the recently active  
https://chemistry.stackexchange.com/q/7971/4945  

I am currently unable to give out a bounty, since I'm busy on a business trip. I find the question quite compelling. The current answers, however, offer little to no insight into which properties of the compound are actually responsible for the effect. For a better understanding of this I think an answer must go deeper than only the chemical structure of a few functional groups and investigate the mechanism of binding to the receptors.