Earlier today I read an article giving five bad reasons to leave a church, and for the most part the article was right on. Most of the excuses were in fact focused on personal taste and "what's in it for me?" rather than on theology and service -- that is, the worship of God and doing whatever small part people can do to make him known.
That said, of course, there are legitimate reasons to leave having nothing to do with selfishness; in fact, leaving a particular assembly may be necessary for continued spiritual growth and in fact charitable. Here are some:
1) Theological infidelity. If someone stands up in the pulpit and rejects the counsel of Scripture or suggests that knowing Jesus isn't all that important -- that he or she doesn't believe in the supremacy of Christ or that the Bible represents ultimate truth -- run, don't just walk, away.
2) Moral failure in leadership. I'm talking about not just pastors, either; if even lay leadership uses its position in the church for its own benefit the members will reflect that, and no one grows as a result. Inappropriate sexual conduct and misuse of money represent only examples of such.
3) A focus on "traditions." There's nothing wrong with traditions in and of themselves; however, they become a problem when God wants to move away from them, which is His perfect right. I know of at least one church that shut down completely and another that's still limping along because they simply wouldn't -- or couldn't -- move with the Spirit of God.
4) Ministry opportunities. You may be very satisfied where you attend, but God may call you out to do something different (such as a plant or even a foreign mission). In that case, staying in your church represents disobedience.
And even if you do leave, make sure you do so graciously, with malice toward none and, if any of the first three apply, even with tears -- because if you truly love the church you're exiting you will need to grieve. But on the other hand, God will reward your obedience with something better.