What is ideal about Gatsby's dream? What is corrupt?
In The Great Gatsby, Gatsby's view of and love for Daisy is idealistic from the beginning.
Nick, recalling Gatsby's courtship from five years before the novel's present, says that Daisy was the first "nice" girl Gatsby had ever known; Gatsby "had never been in such a beautiful house [Daisy's] before"; there was a hint in her house of "romances that were not musty and laid away already in lavender but fresh and breathing and redolent of this year's shining motor cars and of dances whose flowers were scarcely withered"; she was highly sought after, so Gatsby "took what he could get, ravenously and unscrupulously--eventually he took Daisy one still October night, took her because he had no real right to touch her hand."
Gatsby puts Daisy on a pedestal from which he never removes her. Even after she allows him to take the blame for Myrtle's death and chooses to stay with Tom, he spends the next morning waiting for a phone call from her.
And his ideal is corrupt as all ideals are corrupt: they are unrealistic; too good to be true, as the cliche goes.
The ideal aspects of Gatsby's dream lies in his belief that with more money and greater social ascendancy, he can find happiness. Whether this happiness is with someone or within his own belief that the trappings and attributes of wealth can help to define a sense of contentment in his own consciousness is what is ideal about his dream. In my mind, the obvious corruption is the underhanded dealings and individuals with whom Gatsby collaborates in the hopes of achieving it. I would also suggest that Daisy, in her own state, might be a form of corruption. I think Gatsby understands that her predicament is one where she is loyal to money. This would explain why she is with a buffoon like Tom. However, Gatsby believes he can either win her over or change her sensibilities, and in this, the corruption lies. Daisy, in her own right, represents a corrupt aspect of Gatsby's dream because she lives and resides in a social setting where individuals like she, Tom, and Jordan all represent social forces that can only corrupt and corrode, and not create.
I think what is ideal about Gatsby's dream is his own belief in the purity of what he is pursuing. To him, he is pursuing an ideal that is perfect. He does not look at the object of his dream (Daisy) critically -- he idealizes it instead. The fact that he feels this way gives his pursuit an noble quality, in my opinion.
However, even though he is pursuing something that is (he thinks) pure, he is willing to pursue it in corrupt ways. He does not care if he gets his money through fair means or foul. He is willing to do illegal things to get the money that, he hopes, will make Daisy love him.