I know this has been discussed around the blogernacle, but I just wanted to share a few historical anecdotes.
The first time I read the Nicene Creed (on my mission) I thought, “do we really disagree with this?” This thought has only been compounded as I’ve studied Christian history. Despite this initial reaction I still had the assumption that those who challenge teachings on the Trinity must be kindred spirits because I had always been taught that Trinitarians believed that the Godhead (Trinity) were all the same person. Not so. Michael Servetus famously “denied the Trinity” in the sixteenth century, but he did so by denying the divinity of Christ. Similarly Edward Wightman also got in trouble for denying the Trinity in the early seventeenth century. Wightman is an ancestor of mine and we often heard stories of how he valiantly went to the stake for his beliefs. Later I found out that not only did he deny the divinity of Jesus but he also claimed that he himself was the Holy Ghost.  My favorite Trinity denial comes from Elisha Paine who was accused of denying the Trinity in 18th century colonial America “for he could not conceive of three Persons … unless making them three Gods.” C. C. Goen points out that Paine was promoting modalism: the Godhead is all the same person.  Thus all these Trinity deniers fall well outside Mormon beliefs.
Nevertheless, it’s true that Mormon conceptions on this point are not perfectly in accord with how the creed has been interpreted. For instanced in the 13th century, the great medieval mystic Joachim of Fiore was condemned for his Trinitarian conception. Joachim taught that there were 3 ages of the world: one of the Father, one of the Son, and a coming age of the Holy Spirit. After his death, he was accused of technically dividing the Trinity more than one should. This reminds me of a point made by Douglas Davies about the Trinitarian implications of the Mormon degrees of glory: “The grading of heavens in terms of the divine personage present in each is in marked contrast to the unified focus grounded in the worship of the undivided Trinity of traditional Christianity.”  Yet Joachim’s supporters claimed that he was condemned on a technicality and his teaching continued to enjoy considerable popularity throughout the middle ages. Similarly, the Mormon division isn’t that far off. 
Another telling instance comes from the vision of Grace Murray, an early Methodist who had a vision of “God the Father looking upon me through his Son, as if I had never committed any sin. I saw the Son as one with the Father, and yet distinct from Him.” Says John Kent, “This was a sound, orthodox vision … which she partly described in words that had been taught…. These events were being recorded after an interval of eight or nine years, however, and they were transmitted through John Wesley: they were therefore twice edited.”  Joseph Smith and his followers, though quite similar in their beliefs, never bothered with such orthodox editing.
 Ian Atherton and David Como, “The Burning of Edward Wightman: Puritanism, Prelacy and the Politics of Heresy in Early Modern England,” English Historical Review 120:489 (December 2005): 1221–23.
 C. C. Goen, Revivalism and Separatism in New England, 1740-1800: Strict Congregationalists and Separate Baptists in the Great Awakening (1962, reprint; Middletown Conn.: Wesleyan University Press, 1987), 120.
 Douglas J. Davies, An Introduction to Mormonism (Cambridge: Cambridge University Press, 2003), 84.
 I’ve often heard it claimed that the “one substance” phrase was at odds with Mormon teachings but I think this is a misunderstanding. To quote Davies again, “the Holy Trinity’s members being of ‘one substance’ with each other sets them apart as ‘God’ from the ‘human substance’ of being human…. For Latter-day Saints this is not a problem, for very individual shares the same ‘substance.’” Davies, Introduction, 80.
 John Kent, Wesley and the Weslyans: Religion in Eighteenth Century Britain (Cambridge: Cambridge University Press, 2002) 123-24.