Well, each coven has its own rules and own culture but there are a few blanket things that I think most covens are looking for. Explicit rules for each coven should be found on their homepage or within the coven.
First, most covens I know look for people who fit in with existing members. Having similar interests helps but individual personalities react to each other in different ways. Being friendly and respectful never hurts.
Honesty isn't a bad thing either. Depending on the coven, members can get quite close and it can be very disappointing to find out the person you've been talking to for months has been lying to you. Also, coven leaders will probably be keeping an eye out for plaigerism and issues like that rarely go down well.
Also, if you'll be gone for an extended period of time most covens ask that you let someone know. Outside of being generally polite though there aren't many unwritten rules.