I think it'd be hard to attribute heresies to Protestants considering the fact that there something like over 10,000 various Protestant Denominations.
That's the problem here. When talking of the beliefs of Protestants, or even Evangelicals, one can only talk of trends or tendencies, not universally held beliefs, since there is so much variety and disagreement within that grouping.
That said, one tendency that I have noticed among some, but not necessarily all, Evangelicals, is that their efforts to denigrate the Mother of God often lead to Christological heresy. I've heard more than one Evangelical say that "The Virgin Mary was only the mother of Christ's human nature," and therefore can't be called "Mother of God." This of course results in a sort of Theodorean Christology, which separates Christ's humanity and divinity in a way that is condemned by our Third Ecumenical Council. Whenever I've encountered an Evangelical who tells me this, I usually respond by asking them if they believe the Virgin Mary gave birth to a baby or a nature. When they respond that she gave birth to a baby, I ask if they believe the baby was God. The conversation goes on from there, until the Evangelical finally understands what we mean by "Mother of God," and admits that in that sense she really is the Mother of God; or the Evangelical will go the other way and display an understanding of Christ that really does separate His natures in a way that can be condemned as heretical.
Another thing I have seen, but to a lesser extent, is the claim that the Virgin Mary can't be called "Mother of God" because Christ's flesh was a "heavenly flesh," and that he didn't even get his humanity from her. That would be docetism, or some other quasi-Gnostic heresy. These people won't even call her "Mother of Christ," because they just don't believe she was His mother in any real sense. Again, I've seen that much less frequently than the other tendency I mentioned above, but the fact that it exists at all within the Protestant communion without being condemned is to me quite remarkable. People will go after us Orthodox for believing that works play a role in our salvation, but they will ignore a heresy among their own brothers that in essence denies Christ's humanity.