I find it interesting, that this was answered only by non-Catholics. The Catholic Church is based on Scripture and Tradition. Tradition being things that were passed down orally through the apostles/priests. Definitely emphasis on the Scripture aspect. I'm sure many Protestants, can and will find things they consider un-Biblical about Catholicism.
As for some of the ideas already mentioned.
Worshiping Mary - Catholics (at least faithful, well informed ones who are following Church teaching accurately) do NOT worship Mary. Catholics describe Mary saying - "blessed are you among women, and blessed the fruit of thy womb Jesus" (Lk. 1:42). Catholics believe that God blessed Mary by choosing her to give birth to the Son, and that she is blessed because of the fruit of her womb - Jesus Christ! This is part of the common Catholic prayer the Hail Mary, which comes directly from Luke. We believe she is special because of this. We do not believe she is God. You know that tight familial bond that we have with our relatives here on Earth? The love a mother has for her child? Well we believe that Mary and Jesus shared this relationship! We believe that Jesus had a special love for his Mama, and that he continues to love her in a special way and listen to her, so we ask her to intercede (pray) TO HIM, for us!
The Trinity - There is a lot to say on this one, but I think I will go with saying that it is not a solely Catholic doctrine. It is believed by a lot of other Christian denominations, including non-denominational Bible Churches, so honestly I barely believe it deserves a mention here. However, I will say that Catholics are fully aware that the word "Trinity" does not appear in Scripture. The CONCEPT of the Trinity, is however apparent, as God the Father, the Son, and the Holy Spirit are all clearly mentioned in the Bible, and are all represented as the True God.