Massively increase the deficit, help the super-rich at the expense of the middle-low incomes, etc. Basically what you'd expect from a Republican tax proposal.
And apart from anything else, isn't having a massive tax spike well... stupid? It can genuinely damage incentives. If getting a pay rise would suddenly cause you to pay 15% more tax then maybe you'd want to avoid it. Wheras with a smoother tax rate rise you'd take home more money for a greater salary in all cases, meaning you'd have an incentive to get a pay rise at any income level (although Republicans might maintain that the fact that you're losing 35% of it to tax would still magically demotivate you).
fakeedit: Beaten, but to elaborate a little bit n case someone for some reason doesn't want to read Aqizzar's links: tax rates are marginal, that's why they're rates. That means if, say, you have a $0-$100,000 tax bracket at 10%, and a $100,000+ at 25%, but you have an income of $1,000,000, you pay only 10% on your first hundred thousand (for $10,000 in taxes there), and 25% on the rest (for $225,000, and $235,000 total). If you earn $100,001, you pay 10% on the $100,000 and 25% on the $1.
This example is highly simplified obviously, but that's basically how taxes work. Being in that 25% bracket wouldn't mean you pay 25% on your entire income, just the part not covered by the (presumably) lower brackets.