for me its
1. location. (rehearsals for me normally mean me going straight from work so I don't want to have a 2 hour journey and then have to get home again)
2.age ( nothing against age at all im 38 but I cant see me having much in common with people in their early 20s when your sitting around waiting at gigs,travelling etc)
3.Travel ( I share a car with the wife so id like people that have ways and means of getting about comfitably)
to add to 3. I don't mind giving people lifts, its just with some people it becomes known that YOU drive them everywhere.