I'm not certain I follow your implication from the first part of your question. Since it would be important for the church to agree on some basic theological (and here, Christological) principles, then surely before such issues were worked out, it stands to reason that certain positions which were taken up during a period in which such clarity did not exist would end up being considered heresy.

If you mean to focus more on why contemporaries viewed the position as heresy, I suppose the most important thing to focus on is that the Arians, of course, did not consider themselves heretics, and they formed a significant bloc in early Christianity, such that they did not go away easily. Indeed, the Arians considered the followers of Athanasius's teachings (often called Nicene Christians after the creed established by the Council of Nicaea) to be as in error as vice versa (see this letter from Arius to Eusebius, an Arian; had they eventually come out on top, I have no doubt that the mainline Arian church would have happily denounced Athanasianism as the first great Christological heresy.

If your question is what in particular about Arius's teachings was seen as heretical, then that's relatively simple to answer. Nicene Christians accused Arius of denying the divinity of Christ (which is understood in the modern sense far too simply, I argue -- the Arians did not claim that Jesus was not divine so much as not an unbegotten deity) by calling him a creature of God the Father; the Nicene argument would be that this is blasphemy against the divinity of the Son. Nicene Christianity instead argued that the Son was of "one being" with the Father, thus (to their own satisfaction, not that of the Arians) preserving the unity of the One God.

It's interesting/confusing/maddening that the Gospel of John was of considerable importance to both camps; John 14:28 being a major support to the Arians, while the first four lines of the Gospel are particularly Athanasian.