Odyssey has very few reprints (cards that were previously in other sets), but it wasn't always that way. Many old favorites were slated to be in that set, but ended up being pulled for one reason or another.
Sometimes, a card that was previously "harmless" can become a real problem in a new environment. With the introduction of threshold in Odyssey, the number of cards in your graveyard becomes really important. Some old cards were almost going to be in Odyssey originally because they fit the theme, but playtesting showed that they fit too well, allowing threshold to be attained far easier than R&D wanted. As a result, they were yanked. They were: