Spiritual Feet

Through the washing of feet, what I see is that when we are working in the world we will get dirty. Walking among sin and spending time with sinners does not leave us clean. We ourselves sin, we are not impervious. We will stumble, fall, screw up, and sometimes don't even notice how much we are influenced by the world as we walk in it.
The disciples walked with Jesus in the world and had their feet dirtied by the dust of the roads and the grime of the places they stayed (Jesus was friends with prostitutes and widows and thieves. Where He was going was not clean.) In the same way, even when we are walking with the Lord, our spiritual feet can become dirtied by the world we walk, serve, and work in.
The washing of feet is not just a "serve each other" moment (it is that too, but I see it as even more). What we are to do is to help cleanse each other of the filth that we accumulate while doing Jesus' work on earth. Christians need to get their hands dirty, but then they need to fellowship and serve each other by pointing out the areas that we are becoming tarnished or swayed by the world that we're serving. We wash each other's spiritual feet by being a family of accountability partners, watching each others' backs, making sure our worldviews, thought patterns, and habits continue to be God-honoring in the midst of a sinful world.

We are in the world, but we should not be of the world.