From what I recall reading about him in the past, Origen was extremely prolific in his day, and only a small portion of what he wrote was heretical. Most of what he wrote was perfectly Orthodox and at the time of the First Council he was widely respected. In fact, I seem to recall that both sides at Nicea were quoting him and using his writings to support their positions.
I think among other things, the Orthodox at Nicea were using him to support their argument that Christ is eternal. The Arians, I think, were using him to say that Christ was subordinate to the Father. I think Origin referred to God the Father as "The God," but he referred to Christ as only "God," or something like that. (Of course this would all be in Greek, which I don't know.) I'm not sure if Origin meant to make Christ subordinate to God the Father, but the way in which he referred to them led the Arians to think he did. I guess that, coupled with other things he taught, like the preexistence of souls, led to him eventually falling out of favor. Most of what he wrote, though, from what I understand, was perfectly fine.