What is the "Filioque?"
Filioque is the word that was inserted in the Western version of the Nicene Creed to assert the doctrine of the procession of the Holy Ghost from the Son as well as from the Father. This was an innovation in the Western Church and done without an Ecumenical Council. It was one of the central issues in the Great Schism of 1054. It is still a difference in doctrine between the East and the West.
Why is this change so significant?
It is precisely because it changes the nature of God. It upsets the balance of the three persons of the Holy Trinity. If the Holy Spirit proceeds from the Son then it puts the Son in a superior position to the Holy Spirit. Traditional thought about the Trinity is that for any given trait, it must be either common to all Persons of the Trinity or unique to one of them. Thus, Fatherhood is unique to the Father, while begottenness is unique to the Son, and procession unique to the Spirit. Godhood, however, is common to all, as is eternality, uncreatedness, and so forth. Positing that something can be shared by two Persons (i.e., being the source of the Spirit's procession), but not the other, is to elevate those two Persons at the expense of the other. Thus, the balance of unity and diversity is destroyed. Many view that this change undermines the role of the Holy Spirit in the Church. Thus, with the role Spirit being denigrated, his traditional ministries are effaced or replaced. The Church's unity becomes dependent on an office, spirituality becomes adherence to the letter of the law rather than its spirit, sacraments come to be understood in terms of validity, and a spirit of legalism prevails. This is the orientation that did develop in the Western church.