First, your 100,000 and current rate at 25% is a distract or. You imply (first) the current tax is 18.75 (18,750 of 100,000). Actually, if the 18,750 were correct, that'd be off of an AGI of 75,000 (in your simplified example). But, SURPRIZE, the fed income tax is progressive, so (according to 2011 tax table for married joint-- the 25% rate for 100,000) you current tax would be a little less than 11,000. So, from your income of 100,000 it'd be about 11 %.
Um, what was your point? That taxes are too complicated for you to discuss cogently? Point taken.