Overview of Critics' position

Although the Church of Jesus Christ of Latter-day Saints says the prophet (president) of the Church speaks directly for God himself, there have been numerous instances throughout the history of the Church where a prophet made incorrect prophesies, or where he taught things as eternal principles which were later changed or voided. For example, many teachings given by the prophet Brigham Young, often in General Conference, are now specifically denied by the LDS church. If prophesies can be false, and doctrines given by one prophet can be invalidated by another, what good is a prophet? If the prophet is only speaking as a prophet sometimes, and as a man at other times (over the pulpit in General Conference), how is a member of the church to tell the difference?

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Problem summary

No prophet since Joseph Smith has really prophesied, acted as a seer or provided any significant revelation, except things like reversing Joseph Smith's polygamy revelations or reversing Brigham Young's (and succeeding prophets') ban on blacks receiving the priesthood. If we, as Latter-day Saints, believe Joseph Smith restored the gospel and was the first in a long line of prophets, then why have prophets since then not exhibited all of the same gifts that Joseph demonstrated and that the current Church teaches that existing prophets should have?