I've heard and read in "Worlds Made by Words: Scholarship and Community in the Modern West" (book by Anthony Grafton, page 222) that Origen was considered a Church Father but not a Saint by either the Roman Catholic or Eastern Orthodox Church? Why is this so?
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.
From this article in Christianity Today, for example:
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