I'm not certain why Limbaugh is the focus of such vitriol. He is simply the son of a conservative Midwestern lawyer and whose grandfather was a well respected conservative minded judge. Limbaugh simply believes in: Milton Friedman, capitalism, free markets, rugged individualism, limited government, conservatism, Ronald Reagan, original intent of the Constitution, liberty, and the founding principles authored by the US Founding Fathers. Similarly he despises - liberalism, communism, socialism, Keynesian economics, big government, crony capitalism, social welfare, and media bias.
What he does on his program is simply read the headlines of the day and then comment. Often times he is making analogies associated with the thought process of liberals and their socialist ideology. Now liberals find that offensive because it makes liberalism look so pathetically shallow and contradictory. Limbaugh merely points out the absurdity of liberal ideology. And in doing so Limbaugh is quite funny, which is a character trait liberals lack and is one thing that infuriates the left wing.
The great thing about Limbaugh is he cares little about political correctness. And the left has done everything in their power to limit his power in the media. Of course their efforts have met with failure, just as has their liberal ideology met with abject failure globally. And as such, instead of looking inward to their political ideological philosophy, they attempt to destroy conservatives as a way to divert attention from their own failings. And even though the left claims not to take him seriously, there actions are those of people obsessed and frightened of Limbaugh's clout.