In his book The God Question, JP Moreland sets out to give a basic overview of why someone should consider becoming a Christian and what her life should look like in following Jesus.
In his chapter on Worship he states:
Christian philosophers call God a “Maximally Perfect Being.” This sounds pretty heady, but in reality, it is a crucial concept. To see why, think of people who are phenomenally gifted. Now, these folks deserve respect for their attributes, abilities, or whatever. How much respect? They deserve a degree of respect proportionate to their excellence in their area of giftedness, such as intelligence. But these people do not deserve our complete or full respect. Why? Well, if someone more intelligent came on the scene, the new person would deserve more respect. So even if a more intelligent person is not around, we know such a person undoubtedly exists, so we hold back our respect a bit.
A Maximally Perfect Being is one who could not possibly be surpassed in wisdom, mercy, love, power, and so on. God is not the greatest being who happens to exist. He is the greatest being who could possibly exist. The implication should be clear: God is worthy of our complete, full, deepest, total commitment. Worship is the act of giving admiration, respect, affection, honor, reverence, and adoration to God. And given the nature of God, worship should be unreserved and total.
I think his thoughts give a good overview of why Evangelicals are so scandalized by Mormon teachings on the nature of God. To view God as someone who may have once been a man, might have been a sinner, or even someone who is in the process of progressing robs Him of something integral to His character. To the Evangelical ear, Mormonism states that God is not a Maximally Perfect being.
This is part of the reason Evangelicals often ask Mormons why they don’t seek to worship the god of God. We want to give respect and worship to the one who deserves it most. We are seeking out the greatest possible being. If there is one greater than Heavenly Father we want to offer him our praise.