If you want to put yourself into a serious depressive state, read the comments section of any remotely controversial article on the internet. Doesn't matter if it's People Magazine or The New Yorker - the commenters are brazenly stupid. On Christian blogs and publication sites, you'll often come across shouting matches between thoughtless legalists and thoughtless libertines about whether or not a certain practice is appropriate for Christians. Eventually, the legalist plays what he thinks is the trump card - "But if we do this, then how are we any different from the rest of the world?" Subtext: And more importantly, how will I look different from the rest of the world?
Of course, Christians ought to live very differently from the rest of the world, seeking to walk each day in the presence of God, to live in gracious freedom from sin, and to work for the renewal of God's creation. But the mechanics of our lives are necessarily going to be very similar to those of other people, regardless of our beliefs. American atheists eat three meals a day, but there is no particular reason for American Christians to eat four. It isn't possible to determine that an isolated practice is sinful simply on the grounds that the world does it.
Now, I do think that most of our practices, even the mundane ones, should be infused with recognition of God's grace. We may eat same food as the unbeliever with thankfulness to God, but we eat with thankfulness to God. We speak the same language, but we speak truth. Our excretory activities, however, are probably identical. We can use the idea that we must be different from the world as incentive to evaluate how and what we do, but it does not by itself say anything about how to make that evaluation.