Furthermore, there is a stabilizing benefit to a progressive income tax. When times are good, more people enter into the high tax bracket, and speculation is discouraged. When times are bad, the tax is automatically scaled back for people whose incomes have dropped. The effect is a calming one, since a booming economy can be restrained, and a lagging one can be kick started (with perhaps some additional tax cut or spending stimulus). I am all for getting rid of as many tax credits as possible, as I don't think the gov't should be telling us what to do. I would prefer a lower tax rate to my mortgage deduction. I think the gov't should tax behaviors that are basically considered universal negatives (smoking, pollutants, etc), but that they shouldn't actively encourage this or that specific behavior by paying us for it. For example if they want a fleet of cars with better fuel economy, raise the gas tax, don't give the guy with a Prius a tax credit. That's my two cents on taxes.
As to mortgages and charities, it has been studied that getting rid of mortgage deductions would actually spur homeownership because the people have more disposable income in their pockets, or at least know what their tax expenditures are to better plan a savings plan for down payments. The same could hold true for charities. I have looked at plans that leave only those two in place, and that may be a good compromise, but cut all the other deductions and loopholes.
Charities are a tough one. If I didn't get a deduction for the couple hundred bucks I give to charitable organizations every year, I would still give. Its such a small amount of money that I don't really care too much about the tax savings. But a lot of charities or non-profits are sustained by massive contributions from very wealthy people. For instance, in the hospital in which I work there is a public area that honors anyone who has given more than $1,000,000. These people absolutely would not give that kind of cash if it weren't tax deductible. It would be harmful to a lot of good organizations if the charitable deduction were taken away, but you can't really say that my $100 isn't deductible but her/his $100,000 is. For that reason, I would leave the charity deduction. Europe is a good example. They don't have charitable deductions in any Western European country, and the result is that all the art museums, hospitals, food banks, etc. are run by the governments for the most part. I don't like the look of that for the US, and I'm going to go out on a limb and say you probably don't either.