Looking to join an established league.....
My priorities would be friendliness and maturity of fellow owners, frequent activity, competitiveness, a quality commish, stability of the league overall.
If it's a money league, that's cool with me as long as the finances are run through LeagueSafe. I won't just send money to someone I don't know. My upper limit is probably in the $50-75 range, so big-money leagues won't be a consideration.
A slow draft is something I always liked, but a live draft is fine too (as long as I can make whatever date and time is set).
I've seen a few leagues that incorporate weekly NFL picks, and I dig that aspect.
Different wrinkles in scoring and roster management don't bother me, as long as the rules and prize structures are well-known to everyone from the beginning and don't change mid-stream.
If your league has a vacancy and all of the above applies, please give me a shout.