OK, all you smart guys, we know what a marginal rate is. It is still the case that any entrepreneur will have less incentive to earn that 250,000th dollar than she did to earn the 249,999th dollar. Maybe this isn't "rational", but it is empirical: there do exist entrepreneurs who will decide not to earn that 250,000th dollar. Maybe that means they'll each sell one less widget, but it also implies that some of them will lay somebody off. Business decisions are not continuous, and they are extremely path-dependent. A small change in circumstances (not 4 cents, but the analysis holds if someone decides to make $270K rather than $310K) can lead to very divergent results.

Of course, our income tax system (not payroll or sales tax!) is already quite progressive, so this was already the case. Also, if you believe in utility functions, you can imagine that the 250,000th dollar is intrinsically less alluring anyway. But Obama wants to make the system yet more progressive, so no one will be surprised when these effects increase, with negative knock-ons for jobs and growth. It isn't as though business owners have nothing better to do with their time than provide jobs. Few of them will close up shop, but all of them will reevaluate their workload. When they're not getting paid enough for their work, they'll do less of it. Actually, I think most of us plebes are basically the same.