Via Reihan Salam's twitter, the Committee for a Responsible Federal Budget just released an excellent paper on tax expenditures. The upshot is that there is more spending via the tax code than there is revenue collected via personal income taxes.
This is a massive amount of spending that gets little or no scrutiny. This is because they are tax breaks rather than direct spending. They are automatically reauthorized every year, not held hostage in the current budget showdown on the Hill. The GOP currently characterizes closing these tax loopholes as raising taxes, and won't touch them. For some reason, money sent out as a check by the IRS is different than money sent out by the Social Security Administration or Department of Transportation.
Moreover, the biggest tax expenditures are hugely regressive. The mortgage interest deduction is essentially an individual mandate to buy a home, and the benefits are greater the bigger the house. (The cap is currently at a $1m mortgage.) Meanwhile low-income renters (me!) get no break at all. As Reihan says elsewhere, the contours of the tax debate are completely screwy.