Why bother being more productive only to have it taken away, when you can work less and have the government leave the fruits of your labor alone because you've skillfully ducked the threshold of success above which it's no longer considered your own? That's the danger of this kind of thinking.
This way of reasoning doesn't affect the wealthy, who can relocate elsewhere in the world and find ways for their cash to pirouette out of government grasp. It's the middle-class entrepreneurs fighting to make their way humbly and honestly in an economic downturn, and maybe even pull a few others along with them, who are demonized by such an attitude and bear the brunt of it.
That's one of the biggest problems with class warfare: Little effort is made to differentiate between an independent entrepreneur and a silver-spoon magnate. While Obama claims to support extending the Bush tax cuts yet again for those making under $250,000 annually in support of those entrepreneurs, he was attacking their entire mind-set in the same speech, labeling their individual success as a collective product.
The self-made entrepreneur with a few employees and an annual income of $300,000 isn't a multinational corporation outsourcing to China and stashing funds in the Channel Islands. How much does a person like this really owe society beyond what he's already giving back through his efforts? And at what point, in Obama's view, does such a person's success become his or her own?