What constitutes a sound, biblical reason for leaving a church? Or, alternatively, what situations may not seem to have a biblical principle behind them, but are valid reasons for leaving a church?
A good set of questions. Whenever answering these kinds of questions I always begin by noting it would have been shocking to those living in the NT era to have access to more than two or three church congregations in their respective geographic locale. Such a plentitude of options simply didn't exist, nor was it conceivable.
I would says that some valid reasons are:
Continual teaching of false doctrine
Heretical teachings
Failure of leaders to adhere to biblical standards of leadership
Immorality that isn't held accountable
Marked departure from biblical teachings of ordinances
Lack of Gospel centered preaching and evangelism
Some additional valid reasons, that might not be entirely "biblical" but are important might include:
Significant personality conflicts between yourself and the leadership
Your disagreement with leadership decisions that, if you stayed, would cause disunity
Of course the most legitimate reason is
God is clearly calling you away to serve another local church
Clearly these are up for discussion.