A heresy just means that a view is not the "orthodox" view, orthodox coming from Greek ortho-, "right" + Greek doxa, "opinion" (from dokein, to think; see dek- in Indo-European roots). Orthodoxy is then simply a right opinion. A heresy is a choosing of a faction that is not the majority opinion.
As others have pointed out, what makes for a "right opinion" depends on the opinion that one is already convinced of. Also as such, every heretic, by definition, thinks his own opinion to be "orthodox," and those that disagree to be the heretic.
In looking at the broad history of the church, the vast majority of Christians have subscribed to, among other things:
(As a shorthand, I tend to refer to these things as the basis of 'Chalcedonian Christianity' and gone into great length about that in the link.)
Note that "non-Scriptural" is not necessarily "heretical" nor is "heretical" non-Scriptural. Many heresies are actually from Scripture - although are often "peculiar" in their interpretation. As a classic example - both Nestorianism and Monophysitism are both grounded in a reading who Jesus was - they each simply emphasize different details in the life of Jesus.
This base of commonality is a reed or measuring stick against which new or "novel" innovation is typically measured. The point of theology is often as much about consistency as it is about whether or not something is "right," and as such heresy simply means that a position is at odds with what the vast majority of Christians since Chalcedon have believed.
Protestantism as a whole is thus a heresy when compared against the teachings of the Roman Catholic Church, but is rarely viewed as such, because so many subscribe to it. Marcionism, on the other hand, denies the legitimacy of the Old Testament for Christians, and is not something that many Christians since Marcion have pushed.
The point, then, that a theologian makes with "heresy" is not that a heretic is somehow obviously going to hell - only that his views are out of step with the majority of Christian thought.