First of all, you can't beat the price. It's free head-to-head competition, not salary cap crap. I agree the Yahoo public leagues are terrible; too many inactive teams and immature owners. Now I only join private Yahoo leagues that screen out bad eggs. You can go to "available leagues" and search by autopick or live draft. Once you've joined a league, there's an option to delete yourself before the draft so you can evacuate bad ones.
I am very picky with which private leagues I join and can't overemphasize how important it is to choose carefully. Since last year I have also chosen to be a commissioner in one of the maximum limit of 4 leagues.
You can tell by reading league posts which ones have mature adults and which are infested with draft junkies and idiots. I prefer those that draft during the last week of August and first week of September as they tend to attract more serious active owners.
In Yahoo there are little yellow squares next to player's names when important news happens. They source info from Fanball, Sporting News, and AP reports so it's always reliable.