Mormonism does not teach that God is the only god; on the contrary, Joseph Smith taught his followers that there were multiple gods out there, all equally powerful with the God they worshipped. This is one teaching not generally known about because Mormons identify themselves as Christians and traditional Christianity denies the existence of any gods but their own god.
Joseph Smith on the Existence of Multiple Gods
The book History of The Church of Jesus Christ of Latter-day Saints records Joseph Smith saying the following in his 1844 King Follett sermon:
"In the beginning, the head of the Gods called a council of the Gods; and they came together and concocted [prepared] a plan to create the world and people it."
A couple of months later, on June 16, 1844, Smith said the following in another sermon:
"I will preach on the plurality of Gods. ...
I wish to declare I have always and in all congregations when I have preached on the subject of the Deity, it has been the plurality of Gods. It has been preached by the Elders for fifteen years. ...
I say there are Gods many and Lords many, but to us only one, and we are to be in subjection to that one, and no man can limit the bounds or the eternal existence of eternal time."
Brigham Young on the Existence of Multiple Gods
The existence of multiple gods ruling over multiple words didn't end with Joseph Smith. Although it may have been difficult for some to accept, it was nevertheless promoted by his successor, Brigham Young.
Brigham Young's Journal of Discourses records that on October 8, 1859, he said:
"How many Gods there are, I do not know. But there never was a time when there were not Gods and worlds, and when men were not passing through the same ordeals [mortality] that we are now passing through. That course has been from all eternity, and it is and will be to all eternity."
Brigham Young also claimed that the god worshipped by Mormons ruled over multiple words, not just ours. His Journal of Discourses records that he said in a sermon on sin and atonement:
"How many earths are there? I observed this morning that you may take the particles of matter composing this earth, and if they could be enumerated they would only be a beginning to the number of the creations of God; and they are continually coming into existence, and undergoing changes and passing through the same experience that we are passing through.
Sin is upon every earth that ever was created, and if it was not so, I would like some philosophers to let us know how people can be exalted to become sons of God, and enjoy a fulness of glory with the Redeemer. Consequently every earth has its redeemer, and every earth has its tempter; and every earth, and the people thereof, in their turn and time, receive all that we receive, and pass through all the ordeals that we are passing through."
Bruce McConkie writes in his book Mormon Doctrine (1966):
"There are an infinite number of holy personages, drawn from worlds without number, who have passed on to exaltation and are thus Gods. Indeed, this doctrine of plurality of Gods is so comprehensive and glorious that it reaches out and embraces every exalted personage. Those who attain exaltation are Gods"
Monotheism vs. Polytheism vs. Monolatry
Some say that all of this makes Mormonism a polytheistic church or religion rather than monotheistic as traditional Christianity is. That isn't quite correct, though, because polytheism involves the worship of multiple Gods and Mormons don't do that. In fact, in the above text Joseph Smith makes it quite clear that there isn't supposed to be worship of multiple gods when he says "we are to be in subjection to that one."
So while Smith is preaching that multiple gods exist, he is also preaching that his Mormon followers are only supposed to heed one of those gods — the god which he identifies as being the same god which Jews and Christians worship (though they do so incorrectly, according to him).
It would therefore be more accurate to say that Mormons have a monolatrous faith — monolatry is the recognition of the existence of multiple gods while only worshipping one. Joseph Smith relies heavily on Hebrew scripture for his argument that multiple gods exist and scholars have concluded that early Judaism was in fact monolatrous, with strict monotheism only developing much later.
Regardless of whether Mormonism can be called polytheistic or monolatrous, though, it's still significantly different from the strict monotheism of traditional Christianity. Even given the Trinity, which arguably undermines the strictness of Christianity's monotheism, traditional Christianity is still far more monotheistic than what Mormons teach.
It's also not something that most Christians would be willing to accept as a doctrine that's consistent with traditional, orthodox Christianity. This is a major reason why you won't usually see Mormons ever discussing this doctrine with non-Mormons. You definitely don't see prominent Mormons, especially politicians like Mitt Romney, talk about this doctrine when they talk publicly about their religious beliefs.