"What about people who have never said the [sinners'] prayer and don't claim to be Christians, but live a more Christlike life than some Christians?"
In my mind, Bell's single biggest theological flaw lies in a broken understanding of conversion. I don't know his work well enough to pinpoint how or why, but he really seems to think that Christianity is about how you live.
Christianity is about the condition of your heart. A redeemed heart will offer fruit of the spirit, but it's possible to be a "good" person and not truly be converted. When Bell presents this question, he misses the underlying point: salvation isn't about behavior, but about a change that occurs on an "internal" - or spiritual - level.