What core, common beliefs are held by all (or most) Christians? Are there any core beliefs that all Christian traditions agree on? Are there any that most (>90% of) Christian traditions agree on?

If a list can be made for either, are there any caveats?
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.