Origen was a great teacher, but he also had some non-Orthodox positions on Scripture and the faith in general. His teachings were specifically anathemitized by the Second Council of Constantinople in 1553, which inherently means you can't be a saint, since you are condemned, at least according to the Roman Catholic Church.
That said, he was also an ardent follower of Jesus, a prolific and influential teacher, and fighter of other heretics.
- The only reason he didn't get martyred was because his mother hid his clothes to keep him from going out.
- He literally castrated himself because he felt convicted by Scripture
- He wrote vigorously against the Gnostics and other heretics, and even spurned his patron who supported other heretics.
From this article in Christianity Today, for example:
But Origen’s interpretations pushed the boundaries of orthodoxy.
He believed, for instance, in the pre-existence of souls and that eventually everyone, including the Devil, would be saved. In addition, he described the Trinity as a hierarchy, not as an equality of Father, Son, and Spirit. Though Origen attacked Gnosticism, in many ways, like the Gnostics, he rejected the goodness of the material creation. His critics have always complained that in many ways this teacher was “blinded by Greek culture.”
Between the pre-existence of souls and his unorthodox view of the Trinity, he was technically a heretic, albeit one with probably very good intentions. The Coptic church, and his Alexandrian school of thought had many followers, but even they will admit his teachings were mixed.
Obligatory joke about Origien: http://christianity.stackexchange.com/a/6467/1039