As I've taken church history courses in the past, and discussed the various church councils and creed formations of the first few centuries of the church, one of the things I've wondered about is if the filioque clause was a necessary addition to the Nicene Creed. This issue is one that divides Catholics and Orthodox. Some thoughts on this, first from Everett Ferguson's church history textbook:

In an effort to establish orthodoxy, the [Council of Toledo of 589] may have overdone it by adding the filioque clause ('and from the Son') to the creed. Whereas the current form of the 'Nicene Creed' (approved [at the Council of Constantinople] in 381) said that the Spirit 'proceeds from the Father,' the Latin of the creed adopted in 589 said the Spirit 'proceeds from the Father and the Son,' an expansion based on Augustine's theology of the Trinity and designed to emphasize the full deity of the Son.

The clause was later to alienate Eastern Christians, who argued that the Nicene Creed was inviolable and not subject to change and that the addition introduced two sources of deity into the Godhead (292).

I do not think that the addition of the clause is of drastic importance. I think it is a stretch to say that it introduces two sources of deity. However, while it certainly emphasizes further the nature of the Trinity, I do not think that the omission of it denies the Trinity, or its nature. My professor of church history this semester said in a lecture,

[Adding the filioque to the Creed] has the theological impact of putting the emphasis on the threeness of God. The Eastern Church had some reservations about this, however, typically stressing the oneness of God. It boils down to a matter of emphasis.

It is a shame, then, that an issue that is little more than a matter of emphasis should have played such a major role in the most decisive split in the history of the church.