Depends. 

Given the nature of RP and how not all people stay involved in a site or a story, for me, I think its important that the ad feels flexible enough that I can create some relevance and importance beyond the connection of the wanted ad, so that it doesn't feel like I need to toss a character because the other person left.

I prefer things that range from loose wanted to general concept - and that is to say something like a general idea of personality, gender, etc is fine -- but I don't like feeling like the other person wrote my app with details. 

I tend to see family roles get snatched up, friend groups, and general group wanted. My most successful wanted ads are usually family.