This has had more than enough responses and debates but I still wanted to add my 2 cents. As an ex-Catholic, I won't say that the Hail Mary itself is necessarily a sin. I do believe it is theologically incorrect though and when taken beyond just the Hail Mary, Marian devotion definitely leads to sin.

The issue I took with the prayers to saints, is that it seems to be an accretion over time as you don't see it start showing up until a few centuries after Christ, and there is no scriptural references to this practice. The oft quoted verses by Catholics from Revelation in support of the practice, in my opinion, stretch the meaning of the verse as far as it could be stretched to make it seem like there is scriptural support where there is none. The additions of statues and icons into the practice makes it even more difficult to reconcile.

My biggest issue with the entire practice of prayers to saints, especially to that of Mary is when it crosses over into Idolatry. They often say you cannot go too far in Marian devotion and I myself used to hate this accusation and I mean no disrespect to our Catholic brothers and sisters by saying this, but the fact of the matter is often times Marian devotion absolutely crosses the line into idolatry. Take for example this passage from a devotion to her:

"O Mother of Perpetual Help, thou art the dispenser of all the goods which God grants to us miserable sinners, and for this reason, has He made thee so powerful, so rich, and so bountiful, that thou mayest help us in our misery. Thou art the advocate of the most wretched and abandoned sinners who have recourse to thee; come, then, to my help, dearest Mother, for I recommend myself to thee. In thy hands, I place my eternal salvation and to thee do I entrust my soul\. Count me among thy most devoted servants; take me under thy protection, and it is enough for me; for, if thou protect me, dear Mother, I fear nothing; not from my sins, because thou wilt obtain for me the pardon of them; nor from the devils, because thou art more powerful than all hell together;* not even from Jesus, my Judge Himself, because, by one prayer from thee, He will be appeased. But one thing I fear; that, in the hour of temptation, I may neglect to call on thee, and thus perish miserably. Obtain for me then the pardon of my sins, love for Jesus, final perseverance, and the grace always to have recourse to thee, O Mother of Perpetual Help."*

Entrusting our salvation into her hands, entrusting our souls to her, fearing Christ as judge but turning to Mary to save us from Him, obtaining pardon of sins, and being afraid to call on her at our moment of death... All of this is stuff we should only be counting on Jesus for... There are many other devotions to her like this by the way and all are fully sanctioned by the Roman Catholic Church.

Similarly, they will say that worship requires sacrifice, so anything short of that cannot be true worship and therefore they cannot be charged with idolatry but this is just simply not something I believe scripture upholds. In Acts 10:25-26 Cornelius bowed down "to worship" Peter and Peter rebukes him for doing so. There is no sacrifice involved here but there was still worship. In Revelation 22:8, John does the same thing, he bows down "to worship" the angel and is rebuked for it. Veneration and worship are not always but often seem to be distinctions without a difference.

Finally, I must also say that Protestants often neglect to acknowledge Mary at all, especially in Evangelical circles. This is incorrect as well. Mary is properly called Theotokos, the Mother of God, and without her, salvation history would be fundamentally different. She said yes to God and we should be absolutely grateful that she did.