In many of the earlier poster's defense, the question is why one would hate Rush. An answer like "He's fat," while lacking legitimacy, is quite clearly a reason. An even better one is that he is egotistical, arrogant, and often quite disrespectful to others who have a legitimate point simply to entertain his fans. But I think the OP desires a deeper explanation than that.
Nor do I think that the best explanation is that most people disagree with Rush. While this may be statistically true, (I refer you to Larry Bartels) simply because he has rather radical points doesn't justify people disliking him. Nor does the fact he is a conservative necessarily entail hate; to hate that political alignment blindly would involve hating American leaders from Jefferson to Reagan.
There is nothing wrong with being conservative or liberal, as long as that view is constructive to the betterment of society. Hibbing and Theiss-Morse point out in "Stealth Democracy" that the majority of Americans are truly concerned with the process through which government acts rather than policy outcomes. Quite simply, people want good government far more than they want big or small government. And Rush blatantly throws himself against this fact by consistently arguing for a series of policy outcomes that he believes are superior while injecting controversy, disrespect, and fear into a society that doesn't want any of those things. Yes, disagreements are one of the prime qualities of democracy, but these disagreements have to be handled with civility of Webster and Calhoun (An example with it's own challenges, to be sure, but go with me) rather than the arrogance and hate that underlie both a Limbaugh and an Oberman.
So you want to know why I dislike Rush? Because he has been on the air, talking to 20 million people for 20 years and not a damn thing has gotten better. If he wants to change this world for the better, then he should stop talking about it and actually do whats necessary to make this place better. Instead, he and all of the radicals on both sides of the aisle seem inextricably attracted to the idea that government should fail before we (To quote Justin Timberlake) swallow that pill called pride.
A couple quick notes:
First, I apologize that my post involves what are most likely unread and recognized books, but I have the feeling the OP wanted a marginally educated response. I hope I at least fulfilled "marginal."
Second, I of course realize that Rush does not want the government or Obama to fail. He simply want's Obama's so-called liberal reforms (Like taxing the richest of us, who are far richer in relation to the rest of the society than they were 50 years ago) to fail. Of course he does, hes invested his career on the simple idea that hes right and liberals are wrong. If liberals end up being right, Rush is pretty well...use your own expletive and body part. But Rush should know better than to say something like "I want the President to fail." Consider what would have happened if a Rachel Maddow had said the same in 2002 when our President decided the unilateral invasion of the Islamic world was the best course for our foreign policy. Rush knows its inflammatory and he said it because it was. How could you like that when millions of Americans are hoping that Congress will stop bickering and actually help us?