Like Narnian commented, per the definition given by this site, nothing. However, historically most denominations have agreed upon several things.
The first thing that comes to mind is the existence of both God and Jesus. Also, many agree Jesus is God.
Historically, the Nicene Creed and the Apostles Creed have been generally accepted lists of agreed upon Christian doctrine. They basically say
- I believe in God
- God created the world
- Jesus is his son
- Jesus was born to the virgin Mary and conceived by the Holy Spirit
- He was put to death by Pilate on a cross
- He was buried, but he rose again on the third day
- he ascended to heaven
- he'll come again one day
- The Holy Spirit exists
- the church are God's people
- God forgives our sins
- We'll be ressurected one day and have eternal life
Again, I say historically - these documents have existed for practically two thousand years with general assent by 97% of Christians (the Chalcedonians).
But again, I stress, precious few doctrine are agreed upon by everyone. There's always someone who may call himself a Christian who disagrees with anything you say. For example, some people call themselves (I say that will all due respect, I'm not saying their not) Christians because they like Jesus's teaching, but don't even believe in a God.
Since "Christian" comes from "Christ" (Jesus), I think the only thing you can safely assume is that all "Christians" have some level of respect for Jesus.