Per IRS Publication 17 Your Federal Income Tax (For Individuals), page 22:

Married Filing Separately

You can choose married filing separately as your filing status if you are married. This filing status may benefit you if you want to be responsible only for your own tax or if it results in less tax than filing a joint return.

Special Rules

If you choose married filing separately as your filing status, the following special rules apply. Because of these special rules, you usually pay more tax on a separate return than if you use another filing status you qualify for.

1. Your tax rate generally is higher than on a joint return.
2. Your exemption amount for figuring the alternative minimum tax is half that allowed on a joint return.
3. You can’t take the credit for child and dependent care expenses in most cases, and the amount you can exclude from income under an employer's dependent care assistance program is limited to $2,500 (instead of $5,000 on a joint return). However, if you are legally separated or living apart from your spouse, you may be able to file a separate return and still take the credit. For more information about these expenses, the credit, and the exclusion, see chapter 31.
4. You can't take the earned income credit.
5. You can't take the exclusion or credit for adoption expenses in most cases.
6. You can’t take the education credits (the American opportunity credit and lifetime learning credit), the deduction for student loan interest, or the deduction for tuition and fees.
7. You can't exclude any interest income from qualified U.S. savings bonds you used for higher education expenses.
8. If you lived with your spouse at any time during the tax year:
   a. You can't claim the credit for the elderly or the disabled, and
   b. You must include in income a greater percentage (up to 85%) of any social security or equivalent railroad retirement benefits you received.
9. The following credits and deductions are reduced at income levels half of those for a joint return:
   a. The child tax credit and the credit for other dependents.
   b. The retirement savings contributions credit.
   
10. Your capital loss deduction limit is $1,500 (instead of $3,000 on a joint return).
11. If your spouse itemizes deductions, you can’t claim the standard deduction. If you can claim the standard deduction, your basic standard deduction is half of the amount allowed on a joint return.