Nothing. Absolutely nothing. I love my brothers and sisters in Christ, but one of my peeves is how Christians can sometimes get carried away with things. One popular perspective is that Christian's shouldn't celebrate Halloween. I don't get it. How three children dressed as a police officer, Dorothy, and a duck (and one dog dressed as a pirate) can be perceived as "anti-Christian" is beyond my scope of understanding. Granted, we wouldn't allow our kids to "glorify the enemy" by dressing up as devils and witches, just as we wouldn't want them to dress up as Hitler....it's just not in line with our family values.
Anyway, when I first became a Christian, I researched Halloween because I wanted to preserve this fun tradition in our family, but I didn't want to do so if it was not right in the sight of God. I ran across the book "Redeeming Halloween" by Kim Weir & Pam McCune (published by Focus on the Family). This book "set me free" from the shame of celebrating Halloween and made me appreciate the holiday's true Christian roots and how it's a beautiful rememberance of those martyred for the cause of Christ.
According to Weir and McCune (2004), "..the Christian church reserved the day before the religious festival as a sacred time. October 31 was then set aside as an evening of preparation called "All Hallow'een", or "the eve of the holy ones". Together, they created a meaningful time for the church to remember the early martyrs." (p.15)
My perspective is that I don't want our kids to feel like their faith is against "having fun" (which is what dressing up and getting candy is perceived as by children), I want them to experience the freedom and joy of living a life in Christ!